Probably a Blizzard bug, but since there's no real way to report bugs to Blizzard (as far as I can tell bug reports submitted in-game and on the forums are never actually viewed by or conveyed to the developers; there are simple bugs like "someone copypasta'd the wrong translation" that have been around for
years and would take someone literally 10 seconds to fix) you might as well just work around it and not bother trying to figure out why it's happening.
