Grass/mycelium does not convert to dirt under flowing water

When under minecraft:water, grass and mycelium covert to dirt. This does not happen when under minecraft:flowing_water.

To reproduce:

create a flat plane of grass/mycelium and place an area of water source blocks, with the flowing water staying on the plane. Run the tick sprint command.

I would expect any grass/mycelium underwater to convert to dirt.

The actual result is that only the blocks under water source blocks (minecraft:water) convert to dirt.