Will need to look at that. If it has created an issue with that particular topic I did add some code in 7.5.4 that does some sanity checking on the announcements table when you use the Rebuild Topics option in the control panel.
I fixed it that way (hope to remember correct) that I unstick the thread, post a dummy reply to it. The dummy becomes the new first post but be sorted in dates. After this I can merge and move all as it should.