"On my Mac" folders - Emails disappeared, folder shrunk

After having moved a large number of emails from an IMAP server to a local "on my Mac" folder, I started having issues reading those emails. I thus decided to follow commonly suggested advice, so I:

• Rebuilt the folders (through the menu "Mailbox" > "Rebuild")
• And I also repaired the index files by deleting and rebuilding the "Envelope index" files (as explained e.g. here)

Suddenly, however, my two folders that I had under "On my Mac" have shrunk dramatically! (From 11000+ messages to 143 in one folder, and from12000+ to 701 messages in the other folder).

Unfortunately, this does not seem to be just a problem of indexing emails. Those emails seem to have physically disappeared! If I look into the library (under ~/Library/Mail/V7/***long alphanumeric code***/***name of the folder***.mbox) and check the size of the respective mbox folders, these mbox folders are now a few MB large, while before they were several GB large.

Is there any sense to recover what was there before?

And, since this whole set of problems started when moving this large number of email messages from an IMAP server to "On My Mac" — is there a better, safer way to perform this operation, rather than just manually dragging the messages from one location to the other one?