IMessage randomly deleted till a specific date

This issue has occurred twice. For example, today is October 23rd, yet messages in my IM application have been deleted up to a specific date, October 12th. I am unsure how to recover these lost messages.

I have ample iCloud storage to ensure that messages are not deleted automatically. I have also confirmed that the settings are configured to retain messages "forever," and that my data is consistently synced and backed up.

This issue seems to affect conversations with a specific contact whom I regularly message via IM. It began occurring after I recently changed my phone.

Please advise on how to recover my missing messages and prevent this from happening in the future. Thank you.