If you "lose connection" for even a fraction of a second during game, the sideboard becomes completely unusable for the remainder of the entire match.

Cheaters are widespread aware of this bug & are intentionally abusing it by lag-switching the opponent at the start of each match for an unfair advantage. Not just a few players mind you, but roughly half of the players in the Best of 3 queue are exploiting this cheat.

This isn't a new bug either, it has been ruining the game for months & WotC is choosing to ignore it.