Email that is not encrypted looks encrypted.
I reported an issue against GnuPG upstream that also affects Mutt 1.9.5.
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. If you are using GPGME, then I guess you have to wait for an update from upstream, because then those details are hidden from you.
See https://neopg.io/blog/encryption-spoof/ for a screenshot and more details.