- Find an old topic.
- Select only the topic and merge to another topic. (Make sure the last post of the 2nd topic is newer than the 1st.)
- It goes to the very bottom, destroying the chronological order.
This is the way it has always worked, and is in no way a bug. You can’t willy-nilly merge a bunch of random posts in date order, that would be super confusing. They retain proper order from the topic they arrived from.