As per the wiki, "Due to being inherently fireproof, magma cubes can spawn on magma blocks."

But they don't.

When I noticed the problem, I created a new world and went into Creative mode on Simulation Distance of 4.

I built a 9x9 platform out of magma blocks, up in the air of a basalt delta, with more than 4 blocks of air above the platform.

I moved 44 blocks horizontally away from the platform farthest edge of the platform. No other spawnable areas were within 44 blocks of me.

I waited for 10 minutes. Magma cubes never spawned on the platform.

To be sure mobs were spawning, I then built another 9x9 platform out of Netherrack, next to the magma block platform with a one-block gap of air between the platforms.

Within a couple of minutes, Magma Cubes were spawning on the Netherrack platform, but still not the Magma Block platform.