I have also had the same issue that all my tags conversation lists are empty even though they have a count, and conversations are now untagged.
I also worry about the migration stating that threads with multiple tags will be organised into a single folder during migration -- what will that folder be, a concatination of the tags? the first tag? the last tag?
this is a major regression of functionality. I don't want to lose my ability to find things that were tagged because they have been placed into a different folder.
As an aside, very keen for the "top up" feature, as I asked a coding question that burned up most of my credit early in the month.