That's a 'lazy mess' case, it seems. Someone receiving an encrypted email should use things like the Thunderbird plugin or proper Mutt config, isn't it? So many solutions there, but none might be actually preventing this shit happen.

Apparently what you saying seems to be correct in every aspect. Some time ago when I still used fakebook I set up sending updates to my email encrypted by some PGP key I've lost (intentionally). After some time (it was probably a couple of years though) the shitters began sending me their regurgitations in plain text.