OK, this appears to be an intentional change, as per bug 1620310. What that bug claims it's trying to implement is, "If the message are ordered by date, go to the latest message without verifying if it's read or not. If it's ordered another way, go to the highest message." What does "highest message" mean? Why the different behavior based on whether the inbox is sorted by date?
If I first click on the Date column to sort by date ascending, and then click on the tag column to sort by tag ascending, and then quit and restart TB78, the message that is selected is the message at the bottom, which happens to be the oldest (by date) message in my inbox. If I do the reverse -- click on Date twice and then click on Tag twice -- and then restart TB, the message that is selected is the same message. So for some reason TB thinks that the oldest message in my inbox is the "highest" message. How does this make sense?
The change in bug 1620310 may have made sense for people who sort by date. It does not appear to make sense for people who use any more complicated sorting than that.