Old Messenger chat disappeared after “security upgrade” - how to restore

Hi, I’m looking for help because I think I lost an old Messenger conversation after Meta’s end-to-end encryption changes.

A while ago, I had Secret Conversations (with disappearing messages) with a few people. Messenger showed notifications saying something like “your primary chat will be archived due to security upgrades”. I didn’t delete anything manually.

Today I opened Messenger and my original chat (years of history) is completely gone. Instead, I only see a new encrypted chat with the same people. The old conversation is not in the main list.

What I want is the old, non-encrypted chat with full history, not this new empty/short encrypted one.

Is there any way to restore or access the old chat?