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.
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I think this works as intended as those chunks are chunks assigned with guardian spawnable. And they will spawn on the highest layer of blocks possible, relatively the lava blocks, because there is no block above it but air. I might be wrong.