Hi,
in game #204019 in Schneizel's turn 21 there was at a moment 3 empty pile but at the end of his turn only 2 (an encampment set aside during his turn go back in the supply). The game haven't finished and with one more turn I won the game.
I am not sur about the rule if the game has to end after his turn 21 because at a moment there was 3 empty pile (so it was a bug) or if the behaviour was correct with an other turn for me.
It's a rule. Game end is only checked at the end of a turn.
It can't end in-between-turns either (suppose you buy Donate and trash a Catacombs to empty a 3rd pile)