The messages are likely sync messages, e.g. if you pin a chat they are sent. As for not being able to decrypt them, the reason is likely that Thunderbird still does not support SEIPDv2:
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 …
You can try to decrypt with GopenPGP or rPGP or Sequoia PGP: OpenPGP interoperability test suite
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