The bug

From https://www.minecraft.net/en-us/article/minecraft-snapshot-21w05a

Small dripleaf needs moisture, so it grows on clay or underwater

It is only able to grow underwater if it is also on top of clay, grass, etc. However it cannot be placed on gravel or sand which comprises most of the blocks generated underwater. The wording makes it sound like it should be able to be placed on anything underwater.