As per the long title.
I think when you split a topic, both topics should be in the lists (for bookmarks and subscriptions. So far only the one, having the old id is kept.
So you might think "oh the discussion ended" just because the topic got split and you are no longer subscribed to the content.

And same other way round for merging. cause now only the ones retain their subs. and bookmark who have the topic with the lower id in their list.