Skeleton spawning in 1.5 block height

Built a spider farm, went AFK, checked collection chest to find bones & arrows, inspected spider farm to determine the cause, found no intended game mechanic reasoning behind skeletons being present, however, found a skeleton still alive that was capable of spawning in a 1.5 block height, with it’s head in the roof slab, not taking damage nor capable of detecting me, not sure if skeleton spawned on roof and fell thru what would be a “top” slab, or if the skeleton spawned on the floor despite the top slab making the spawn area 1.5 blocks height, therefore the skeleton should not be able to spawn.

No game altering client, datapack, modification, etc.