The mails you cannot decrypt are probably messages sent to self that are encrypted using SEIPDv2. The issue about supporting SEIPDv2 in RNP, the library used by Thunderbird, is at Implement support for v2 SEIPD packet. · Issue #1770 · rnpgp/rnp · GitHub
The To field can always be completely fake, checking the To field does not tell you what actual recipients are. This is how Bcc works, by sending messages to additional recipients not included in the To field. But it can also be the other way round, you can include recipients in the To field but not actually send to them.
The reason for hiding the recipients in the To field is to reduce metadata when sending to multiple servers. If you are on mailbox.org and send to example.org and example.net, you do not need to tell example.net that you have also sent copies to example.org and vice versa. This is achieved by not disclosing the recipients in the unprotected To field. All the recipients are included in the encrypted part in a protected To header using RFC 9788.
As for “only fetch from delta chat folder”, the option is likely to be replaced by a per transport “IMAP folder” setting, this is how the removal draft is implemented: Remove mvbox_move and only_fetch_mvbox by link2xt · Pull Request #7780 · chatmail/core · GitHub
But we don’t recommend making any new setups using it.