Guardians can spawn in lava.

I cleared out an ocean monument in creative to test guardian spawning. It is clear down to y=5. I covered the bottom with lava to keep other mobs from spawning, but I keep seeing guardians spawn in the lava on y=6. There is nothing but air above the lava.

I also noticed this happening in the second image where there is only a 1 deep layer of lava above the signs. The guardians spawn in the lava and then fall down.