Okay thanks to this I realized what the heck is wrong with Optifine. I cant get Metadata from the World when checking for the ability to spawn Mobs, I just always get 0 returned.
Well Metadata 0 is the “Stone” variant which allows spawning of Mobs for obvious reasons. So all other variants of the Stone Blocks are, as you likely know Spawn Proof normally. So what happens if I cannot check for Metadata? Well all Stone Blocks become Mob Spawnable and I cannot do anything about it.
All I can do is detecting Optifine in Singleplayer and warning people in the Tooltip that the Blocks are Spawnable even though they should not be…