I understand your concern @Sam. The reason I chose to disable auto-un-pin for topic watchers is that the author of a topic will be a watcher.
If I’ve authored a topic and had it pinned (particularly as a paid-for pinning) I would expect to see it pinned.
I think the same would go for someone who’s explicitly watched a pinned topic.
How about I change the logic of my solution so that just the author of a pinned topic avoids auto-in-pin for that topic?