Incoming MMS creates a different conversation stream

Start by checking the reported behavior on app version 1.9.1 and tracing how incoming MMS are matched to contacts and conversation threads. The report says the issue is difficult to reproduce and affects one contact, so compare the contact’s number handling with incoming SMS and MMS. Done means incoming SMS and MMS from that contact appear in the original conversation without deleting its history.

Checklist

• I can reproduce the bug with the latest version given here.
• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one bug.
• I have read and understood the contribution guidelines.

Affected app version

1.9.1

Affected Android/Custom ROM version

Android 15 stock Samsung ew OneUI 7.0

Affected device model

SM-G736B xcover6 pro

How did you install the app?

F-Droid / IzzyOnDroid

Steps to reproduce the bug

Difficult to reproduce, as it seems to affect only one contact. I have tried deleting the second conversation completely, but it creates a new one with the same issue.
The issue affects one contact who uses iOs phone. It is challenging to troubleshoot because it relies on incoming messages from a 3rd party. Other contacts on iOs don't seem to display the same issue.

The contact's information is store on vnd.sec.contact.phone, and the number is in the format 04xx xxx xxx (vs +cc 04xx xxx xxx). There is only one number for this contact.
The application has worked normally with the same contact information for years, and nothing has been changed recently including OS updates on my phone. I don't know if there has been a change on the iOS side of things.

I have tried deleting the cache for fossify messages, deleting the entire second contact and having a new message sent in - this did not fix the issue.

In fossify contacts - some contacts don't display when searched by number (eg 0415), but they do display sometimes with fewer numbers. This might be an issue with contacts; it's not happening in the samsung contacts app. I'm not sure if this is related to the issue with messages.

I deleted all numbers from within the troublesome contact. Then re-added them (type: mobile). Messages was unable to link the contact information and instead displayed the primary conversation stream by phone number. I then used "add number to contact" and it created a duplicate mobile number in the contact. The next inbound MMS from that contact came in as a new conversation, showing the mobile number as the label for the conversation (screenshot). I have checked for duplicate/merged contacts within the phone book - there does not appear to be any. I don't have any synced numbers with eg. google etc. There is only one SIM contact and that's a built in telco one.

It would be highly preferable to not have to delete the entire primary conversation, as this has quite a few years' history in it.

Expected behavior

All incoming SMS and MMS to be in one conversation stream.

Actual behavior

Inbound SMS and outgoing SMS are sent to the original conversation that has been present for years. Incoming MMS are redirected to an identical contact.
Sending outbound replies via the second conversation sends and is received normally, but those are now separate as if it was another phone number/contact. Sending via the original contact works normally.

Screenshots/Screen recordings

Additional information

No response