On multiple battles in a row (specifically colo endurance, but presumably quests as well), between matches, the screen frequently (always?) doesn't update from the completed battle at the right time, making it impossible to mulligan.
It seems like the opponent name updates when you hit "continue", but the game field and just about everything else remains on the old, finished battle.
If you hit "continue" again, at that point the game field updates, but it interprets that click as accepting the mulligan, taking you immediately in to the next fight without the opportunity to mulligan.
If you're me, that means you automatically click "end turn" yet again thinking it's "accept hand" and skip your first turn.