@
[email protected] hmm. I agree in the sense that any combination of recipients can be addressed, but the specific term "follower only" (to the exclusion of the public pseudo-user) isn't AP specific... could be wrong on that one.
Either way I do think it's a good courtesy to assume equal or narrower visibility when replying to any post. The specific issue you outlined in OP seems to be a Mastodon bug for sure.
