Not sure if this is a bug or working as intended, but if someone makes a post, then some time after a new Topic is created and that older post is moved to it, the bump date becomes that of the last post added to the Topic. Would it be better if it was of the latest post?
In case you need to visualise it:
Post was made yesterday.
New Topic was created 10 mins ago.
Old post merged to new Topic.
(Topic shows with yesterdays activity rather than those posted around 10 minutes ago)