Hi. I'm not a dev but could I see a screenshot of your Program Files (x86) folder? I might be able to help figure out what you're looking for
I think the problem might be the hitbox of the seagrass is taller than the hitbox for the water and because it's in water the lava can't set it on fire so the lava does nothing.
Probably not a bug. The blue fire burning texture probably hasn't been added yet but they're planning on adding it.
I experienced the same issue. I had to close out my entire game to get rid of it so it is a bug not just experienced by 1 person.
Maybe go into 1.14.1 and remove any texture packs you have on that aren't on by default and go back
You can blur out any information you might not want others to see.