I just check on a pure PlayStore version on my Chromebook and I can reproduce it. In a new empty folder, move 2 messages with the same date and 2 others with the same date but different from the first 2.I can't reproduce this issue in any way. Item decorators should be updated automatically on notify item/data changed and this shouldn't be done "manually".
At that point you should have 2 date headers. (Use group by date and sort on time). Select the third message, the first of the 2 oldest messages that has a header. Change it's importance to high. That will reorder the list, moving that message at the top with his own header, but then the 3 remaining will have the same header, which is incorrect for the last one.