Nested Repeater Fields with Pagination Enabled Do Not Save Correctly

Start with the supplied nested Repeater JSON and reproduce the save flow in the WordPress backend using pagination on both Repeater fields. Compare the nested values before saving and after reopening the page; done means inner Repeater content is retained and displayed correctly. No source file or test is named in the issue, so locating the relevant save path and adding regression coverage will require project research.

Describe the bug
When two nested ACF fields (both of type Repeater) have the option "pagination": 1 enabled, the inner field content is no longer saved correctly. The data appears to be lost or overwritten during save.

To Reproduce
Steps to reproduce the behavior:

1. Create a Repeater field (Parent Repeater).
2. Inside this field, add another Repeater field (Child Repeater).
3. Enable "pagination": 1 for both Repeater fields.
4. Add content to both fields in the WordPress backend.
5. Save the post.
6. Reopen the post and check the values — the inner Repeater content is missing or not saved correctly.

Expected behavior
The values entered in both nested Repeater fields (with pagination enabled) should be saved and displayed correctly after saving the post.

Screenshots or Video
N/A — the issue is data-related and occurs upon save.

Code
You will find attached the exported JSON of the relevant ACF field group that reproduces the issue. (Attach your JSON export when submitting the ticket.)

Version Information:

• WordPress Version: 6.8.1
• ACF PRO Version: 6.4.2
• PHP Version: 8.1.x
• Browser: Chrome (latest)

JSON:

{
    "key": "group_5e67884841724",
    "title": "Fieldgroup XYZ",
    "fields": [
        {
            "key": "field_5e67889ad8b2a",
            "label": "Flexible Content",
            "name": "flexible_content",
            "aria-label": "",
            "type": "flexible_content",
            "instructions": "",
            "required": 0,
            "conditional_logic": 0,
            "wrapper": {
                "width": "",
                "class": "",
                "id": ""
            },
            "allow_in_bindings": 0,
            "wpml_cf_preferences": 2,
            "admin_only": 0,
            "exclude_customer": 0,
            "layouts": {
                "layout_6604371afv5cd7": {
                    "key": "layout_6604371afv5cd7",
                    "name": "layout_xyz",
                    "label": "Layout XYZ",
                    "display": "block",
                    "sub_fields": [
                        {
                            "key": "field_68042e0594df7",
                            "label": "Repeater 1",
                            "name": "repeater_1",
                            "aria-label": "",
                            "type": "repeater",
                            "instructions": "",
                            "required": 0,
                            "conditional_logic": 0,
                            "wrapper": {
                                "width": "",
                                "class": "",
                                "id": ""
                            },
                            "allow_in_bindings": 0,
                            "wpml_cf_preferences": 2,
                            "admin_only": 0,
                            "exclude_customer": 0,
                            "collapsed": "",
                            "pagination": 0,
                            "min": 1,
                            "max": 0,
                            "layout": "block",
                            "button_label": "Add Item Repeater 1",
                            "rows_per_page": 20,
                            "sub_fields": [
                                {
                                    "key": "field_66265bd4e88e9",
                                    "label": "Repeater 2",
                                    "name": "repeater_2",
                                    "aria-label": "",
                                    "type": "repeater",
                                    "instructions": "",
                                    "required": 0,
                                    "conditional_logic": 0,
                                    "wrapper": {
                                        "width": "",
                                        "class": "",
                                        "id": ""
                                    },
                                    "allow_in_bindings": 0,
                                    "admin_only": 0,
                                    "exclude_customer": 0,
                                    "collapsed": "",
                                    "pagination": 0,
                                    "min": 1,
                                    "max": 10,
                                    "layout": "row",
                                    "button_label": "Add Item Repeater 2",
                                    "rows_per_page": 20,
                                    "parent_repeater": "field_68042e0594df7",
                                    "sub_fields": [
                                        {
                                            "key": "field_662b9e12d3a7f",
                                            "label": "Media-File",
                                            "name": "media_file",
                                            "aria-label": "",
                                            "type": "file",
                                            "instructions": "",
                                            "required": 1,
                                            "conditional_logic": 0,
                                            "wrapper": {
                                                "width": "50",
                                                "class": "",
                                                "id": ""
                                            },
                                            "allow_in_bindings": 0,
                                            "wpml_cf_preferences": 2,
                                            "admin_only": 0,
                                            "exclude_customer": 0,
                                            "return_format": "array",
                                            "preview_size": "thumbnail",
                                            "library": "all",
                                            "min_width": "",
                                            "min_height": "",
                                            "min_size": "",
                                            "max_width": "",
                                            "max_height": "",
                                            "max_size": "",
                                            "mime_types": "mp3",
                                            "parent_repeater": "field_66265bd4e88e9"
                                        }
                                    ]
                                }
                            ]
                        }
                    ],
                    "min": "",
                    "max": ""
                }
            },
            "button_label": "",
            "min": "",
            "max": ""
        }
    ],
    "location": [
        [
            {
                "param": "post_type",
                "operator": "==",
                "value": "page"
            }
        ]
    ],
    "menu_order": 1,
    "position": "normal",
    "style": "default",
    "label_placement": "top",
    "instruction_placement": "label",
    "hide_on_screen": "",
    "active": true,
    "description": "",
    "show_in_rest": 0,
    "modified": 1750158065
}