Realm - Mobs spawning on stair blocks

Noticed mobs continuously spawning on stair blocks on a friends roof when playing on realms. I watched as these two spawned because originally I thought that other mobs may have spawned on the ridge-line of slabs and moved down the roof but they definitely just appeared straight on the stairs

Note, there are no full blocks below the stairs (please see pictures)

Should be easy enough to replicate using a basic roof pattern and stair blocks.