several IMAP accounts

I use Thunderbird 140.9.esr and happened to notice some strange behavior a few weeks ago: I have 5 email accounts, all of which I access via IMAP. Immediately upon syncing, some (not all) emails disappear from the inbox of the first account and appear in the inbox of the third account. When I then manually move the emails back to the correct inbox, they remain there permanently.

When I check the accounts via webmail, the emails have indeed been moved from one server to the other. At first, I thought it was a problem with my filters, but none of the filters were set up to move emails from one server to another. When I disable all filters, the problem persists. Could it be that I made a mistake in the account settings? However, the accounts have been set up for several years now, and I’ve been using Thunderbird for just as long. I didn’t notice this error in earlier versions.