Hi ,
This behavior is the same across multiple versions of the engine and appears to be intended behavior that I can see.
When using the LOW Scalability setting for Post Processing the CVar for r.Tonemapper.GrainQuantization
is set to 0 whereas all other settings have it set to 1. You can see what scalability CVars are affected by the quality changes in the documentation here: Scalability Reference for Unreal Engine | Unreal Engine 5.2 Documentation
-Tim