Mobs spawn where they shouldn't in 1.15.1

Mobs have been seen spawning in secure, fully lit areas and on top of blocks where they should not be able to.

While my back was turned, a zombie appeared in a secure room with no dark areas. Later, I saw a creeper spawn in on top of a right-side-up stone brick staircase, which should not be possible. The game does not seem to be checking the conditions correctly before spawning mobs.

Having creepers spawn on stairs is a huge problem for my server because we use stairs to keep mobs from spawning in some decorated areas that don't look good lit or covered with slabs. A lot of work went into these areas and having mobs spawn there is not fun.