Did a little more cross-comparison with #FinalFantasyVIIRebirth#PS5 in headphones with 3D audio for headphones on and off, using my pair of Sennheiser HD 6XX open ear headphones.
Yeah, audio sources at a short distance away from the player character sound off (muffled) and there's some treble falloff that I don't understand (it makes audio sound compressed or aliased). Even the music mix sounds bad.
3D audio off definitely clears up the headphone mix! Super bizarre.