The issue goes something like this:
when you force someone to unfollow you (in your followers list on the settings page), a message is sent to that person's instance. But for a good while now this message hasn't been interpreted correctly, so the remote instance continues to think that this user is following you. So if your (followers-only) message is sent to this remote instance (i.e. because someone else follows you there), it will also be shown to the person that you made unfollow.
once the fix is in place, the unfollow messages will be interpreted correctly, but users you forced to unfollow before the fix will still be in that weird situation where they kinda follow you but kinda not
So be careful with the "followers-only" privacy level
(thanks to Claire for the explanations and to @Courgette for the french summary here https://mastodon.opportunis.me/@Courgette/104614627915551177 that I translated)