Mobs can't spawn on shroomlight and fully charged respawn anchor

Slime can't spawn on shroomlight.Shroomlight can't do damage to mob and it can prevent player open chest.It can be charged by a redstone repeater.Why can't mob spawn on it?

Jack o'Lantern is very similar to shroomlight but mob can spawn on Jack o'Lantern.

 

Mob can't spawn on fully charged respawn anchor either.