ClientSim spawns player objects in OnBeforeSceneLoad, so changes made to Player Objects in PostprocessScene don't persist to the local player's Player Object. Further Player Objects spawned through "Spawn Remote Player" do have the changes.
Please move the player object spawning to later in the runtime so changes made through PostprocessScene can persist to the local player's game object.
As far as I can tell, this is only a problem in ClientSim. It seems to work fine in-game.