Maybe when there’s only one post, you must add the mod to the PM so the current behavior is already correct? Perhaps there’s nothing necessary to do here @techAPJ.
One thing we could do is add a new PM convert prompt for mods, only when there is more than one post in the topic, that looks something like this:
If you don’t reply to this topic before you convert it to a PM, you won’t have access to it. Grant yourself access?