This is the exact problem I just started experiencing after upgrading to MacOs 13.2 and upgrading GPG to match.
I upgraded on ~2/4/23 from Mac Os ~ 12.2
I tried this suggestion (thank you, seemed good), but it didn't work for me:
"Mail app > Preferences [Mail: Settings] > Accounts > Select the account > Mailbox Behaviors > Drafts Mailbox. If this is set to the email's drafts mailbox already, try switching it to another location for a moment to see if that has helped. "
I tried temporarily switching the draft mailbox, and permanently switching the draft box (including to "On My Mac: All Drafts") It is automatically reverting within 2 seconds back to its default of "INBOX/Drafts"
So far, the only workaround (and it's a pain) is to copy all the message I am replying to and start a NEW EMAIL MESSAGE, pasting in the decrypted body of email I am replying to so the recipient knows the context. Then writing my reply, adding a subject (copy/paste prior subject) and addressing it to the recipient.
Apparently this works because when replying to an encrypted email normally, as you write, the automatic draft saving is failing because it can't decrypt the text from the reply. But, if you copy the decrypted email you are reading, then paste that into a NEW email, it is not decrypted, and drafts save as normal as you are composing the email.
A "Hot Fix" is referenced in this thread, but I don't see a link as to where I would find that to download it? Can anyone tell me where that is? I'm guessing it's somewhere on https://gpgtools.org/ (GPG tools dot org)