With the event I purchased a 20 slot reagent bad and both Backstack and the Blizzard UI choke on it.
Storm Battered reagent bag

Blizzard "clean up bags" simply ignores it and puts crafting materials stored in it into normal bags defeating the bag completely.

Bandstack willl get "confused" during sorting.

Stanzilla/WoWUIBugs#349 suggests that the new reagent slot might not be getting appropriate API returns at the moment, which probably isn't helping.

The newest release has fixed the issue.