The original behavior of the "Fix RNG seed per run" option was such that, if you closed the game during a monster fight and reloaded it after restarting it, the monster fight would play with the exact same random number generation as the last time you tried it.

This was, however, pretty hard to spot and caused the option itself to feel pointless.

The new behavior of the option is instead to fix he RNG seed to a known value and state when a new game starts, guaranteeing the same behavior of the RNG generator if you make the same choices in the same order.

Notice that toggling this option to "N" during a run will allow the RNG generator to be reshuffled.