During the final night, on turn 2, the game will stop halfway through the enemy's phase. Specifically after both bosses finish their actions.
The game will not progress to the normal enemies' movement forward.
The game isn't frozen. Animations and music are still going. Fast Forward still works. Can't open settings menu during enemy's phase.
Through some trial and error, it seems that Negative Alterations are to blame. Using NAs against either(?) boss causes the game to softlock. Not using them against either boss allows the game to progress to the 3rd turn.
Not sure if it's only NAs nor if this still happens against the bosses' later forms.