Shroomlight Tears only grow on the bottom of the shroomlight. You can't even use bone meal on a shroomlight block if there isn't a air space under it.

I expect this is unintended because you can find them growing atop the block in the world.

They only grow atop Shroomlight found in the Warped Forest, it's an unnatural growth spurred on by the biome that can also be seen in the Twisted Vines. this is intended behaviour.