Too many sideboarding bugs are in this game.
1. opponent's card back visual effects are stacking with your own cards during sideboarding
2. dragging cards during sideboarding makes cards go invisible
3. sideboarding gets saved between matches
4. if the above happens going to deckbuilding screen will also have your saved sideboarding during that match and your sideboard will also have 0/15 cards in it until you exit the game.

the first 2 has been in the game for more than a year
the last 2 are new with bloomburrow