A colleague of mine is unable to read GPG-encrypted emails sent to them by another member of our team via our mailing list, saying this1. The recipient, unable to read emails, is using Thunderbird v78.3.1 with the built-in OpenPGP system, while the sender is using Thunderbird 68+Enigmail to encrypt their messages. Mozilla's extensive FAQ/HOWTO2 about their OpenPGP integration doesn't seem to have anything about this.
The sender is also using "Per-recipient rules" in Enigmail to specify which keys to encrypt the messages with.