stairs allow hostile mobs to spawn?

in 18w22c i made a house in my survival world and while returning from mining i cam home to find a creeper inside the house when it was completely lit up, i thought maybe i didnt light it enough so i put even more torches, later on i come back to find 2 zombies inside so i said maybe its a bug and i updated the world to 1.13-pre1 and it still happens so u check the light levels in my entire house and i found that some stairs had light level 0 even with torches all around it, after i removed the stairs from the house and replaced them with ladders nothing spawns anymore. also didnt find anything here with this issue so ye c: