For reference, AvatarUnificationMode enables/disables the R6-to-R15 rig conversion system that Roblox is working on, which will effectively transparently run a R6 style rig on top of the R15 system.
The core issue of this bug seems to be related to texture quality being forcibly dropped - R15 rigs have more textures and use more VRAM, which would explain why you’re only seeing this on a R15 rig.