User field not allowing subscription (only unsubscribes)

The issue is that my lists have inexplicably become one-way ratchets, for which I can remove but not add users via subscription fields.

After setting up the Mailchimp module, I had two lists with corresponding fields on user pages (though only visible to/editable by administrators) properly syncing with my site (with webhooks also enabled). Rules were also working properly to automatically subscribe users to the lists depending on role (whenever newly added to the role).

After adding a new rule to automatically unsubscribe users with a different role and taking the site out of maintenance mode, the subscription fields became a one-way ratchet. I could not subscribe any users not already subscribed to a list by checking the box on user profiles (or using the rule behavior), though I could unsubscribe currently subscribed users (both by unchecking the box or using a rule). Once a user was unsubscribed (by unchecking the box), however, she cannot be again added to the list. All of this had been working properly before those changes.

I disabled all rules and returned to maintenance mode; neither change had any effect. Making a brand new list in Mailchimp with a brand new subscription field works (at least using limited mergefields), but if I duplicate one of the lists exhibiting the issue (so as to preserve the old mergefields) and link that to a new subscription field, the issue presents again.