Water Does Not Make Valid Underwater Plant Locations Below

Pardon my terminology, but it seems as if seagrass cannot be placed beneath source blocks (as they are flowing I imagine).

I make water areas by filling the top and removing the filler blocks to have the water travel downward. This does not work well with underwater blocks.

 

 https://www.youtube.com/watch?v=YyJ8wjeCrOw