Mails that are not encrypted, look encrypted
I reported an issue against GnuPG upstream that also affects Evolution 3.28.2.
The original report is here: https://dev.gnupg.org/T4000
The guts of it is that it is possible to create messages that look encrypted, but are not. You can detect this by checking if the status fd output has "PLAINTEXT" lines after END_DECRYPTION. Also, it might be a good idea to not ignore NODATA (although this is not a complete defense).
See https://neopg.io/blog/encryption-spoof/ for a screenshot and more details.