It looks like when a user who replied in topic A has their post(s) moved to a new topic they are losing they are no longer tracking that topic (the new topic, that is). This is when they are not the OP of the new topic.
Wouldn’t it make sense to move/copy the tracking status to the new topic if they’ve had it in the old topic?
There is still a lot of residual weirdness when splitting topics; we have a fair bit left to work on here.
One thing I definitely resent is that posts I have already read in the original topic are marked unread in the new split topic they are moved to.
Been running into this a lot lately. Adding a 
here for v2.1
Not quite yet, @nbianca fixed the PM -> PM case, but Topic -> Topic is still a bit up in the air.