oneOf appears to not work correctly (Being treated like allOf)

Start by reproducing the reported update validation failure with the schema and payload containing VpcLinkId and BaseURI. Compare validation results for exactly one, both, and neither oneOf branches; done means one valid branch is accepted while invalid combinations produce the expected errors.

Version 1.0.5 of the Java plugin

Schema:

{
    "typeName": "OBFUSCATED::ApiGateway::RestApi",
    "description": "An example resource schema demonstrating some basic constructs and validation rules.",
    "sourceUrl": "https://github.com/aws-cloudformation/aws-cloudformation-rpdk.git",
    "properties": {
        "RestApiId": {
            "type": "string"
        },
        "Name": {
            "description": "Api Name",
            "type": "string"
        },
        "AuthorizerLambdaArn": {
            "type": "string"
        },
        "AuthorizerCredentials": {
            "type": "string"
        },
        "Version": {
            "type": "string"
        },
        "SwaggerS3Location": {
            "description": "Swagger File S3 Location.",
            "type": "object",
            "properties": {
                "Bucket": {
                    "type": "string"
                },
                "Key": {
                    "type": "string"
                },
                "Version" : {
                    "type": "string"
                }
            },
            "required": [
                "Bucket",
                "Key"
            ]

        },
        "EndpointType": {
            "description": "Gateway Endpoint Type",
            "enum" : ["REGIONAL", "EDGE", "PRIVATE"],
            "type": "string"
        },
        "ValidateRequests" : {
            "type": "boolean",
            "description" : "Use Api Gateway request validatoin"
        }
    },
    "oneOf": [
        {
            "properties": {
                "VpcLinkId": {
                    "type": "string"
                },
                "BaseURI": {
                    "type": "string"
                }
            },
            "required":  ["VpcLinkId", "BaseURI"]
        },
        {
            "properties": {
                "LambdaArn" : {
                    "type": "string"
                }
            },
            "required" : ["LambdaArn"]
        }
    ],
    "additionalProperties": false,
    "required": [
        "Name",
        "SwaggerS3Location",
        "EndpointType",
        "AuthorizerLambdaArn",
        "AuthorizerCredentials"
    ],
    "writeOnlyProperties": [
        "/properties/VpcLinkId",
        "/properties/BaseURI",
        "/properties/LambdaArn",
        "/properties/SwaggerS3Location",
        "/properties/ValidateRequests"
    ],
    "readOnlyProperties": [
        "/properties/RestApiId",
        "/properties/Version"
    ],
    "primaryIdentifier": [
        "/properties/RestApiId"
    ],
    "handlers": {
        "create": {
            "permissions": [
                "iam:PassRole",
                "s3:GetObject",
                "s3:GetBucketLocation",
                "apigateway:PATCH",
                "apigateway:POST",
                "apigateway:PUT"
            ]
        },
        "read": {
            "permissions": [
                "apigateway:GET"
            ]
        },
        "update": {
            "permissions": [
                "iam:PassRole",
                "s3:GetObject",
                "s3:GetBucketLocation",
                "apigateway:PATCH",
                "apigateway:POST",
                "apigateway:PUT"
            ]
        },
        "delete": {
            "permissions": [
                "apigateway:DELETE"
            ]
        }
    }
}

Error when using only VpcLinkId and BaseURI in an update operation:

Model validation failed (#: #: only 1 subschema matches out of 2)\n#: 2 schema violations found (#)