Mobs like Ghast piglins and zombie piglins are able to spawn on Glowstone, Slime blocks, ice, inside of torches and more.

This blocks are Opaque blocks and mobs shouldn't be able to spawn on or inside of them. This problem ruins big nether bases because it turns out Ghast at really terrible guests (regardless what decorations i use in my base Ghast always try to remove it).