This isn’t a bug, it’s a design choice for performance.
Each planet has a fixed “sea level” and only water at this level will generate reflections. @lucadeltodecso@dave has posted recently on the specific technical reasons if you want to scan his history for it.
I guess I just have to lower the floor of this room by one then, slightly annoying but considering how good water can look in this game, It will have to be done.