This is caused by a known bug in the merge procedure where the forms answers to be merged contain answers prepopulated using the person id uri.
📌Note: To open the link, you'll need to log in to the Customer Success Portal.
The fix for this bug is available from Mosaic 22.2.3.0.
If a customer is not able to upgrade to 22.2.3.0 and urgently needs the fix, the updated merge package can be provided as a hotfix. Please see the notes on the linked ADO case for scripts and instructions.