I just want to point out that this isn’t to do with the water being placed.
There are only reflections on water blocks at the particular planet’s sea level. Most of them have the same sea level I think (64m) but I don’t think they’re 100% identical.
Water blocks you place at this altitude will have reflections enabled. Water blocks at other altitudes will not, even if it’s a natural block in a cave or on a mountain.