I realize now the most likely scenario: Perhaps Endwalker-specific content was designed only for this new, poor sound engine, and so some stuff can't work on the old, better sound engine for whatever reasons, thus they actually can't go back to 5.5 sound engine even as a personal option. What an utter nightmare. Still, they could at LEAST just revert back to 6.0, seeing how 6.0.1 only made it worse.