I would Check-in Changes just in case then delete them all, it should generate a new waterbrush.
The waterbrush holds the info on how a waterbody is calculated with the landscape that holds it.
If you have a landscape, delete it, then make a new landscape with the same name, the old waterbrush doesn’t get deleted but causes conflict with the new waterbrush. Often using the first generated waterbrush to define how the waterbody looks, but using the deleted landscapes terrain info.
I have a project with 4 different landscapes, landscape_1, landscape_2, landscape_3 & landscape_4. I had this trouble after deleting and replacing landscapes a few times to stay in the landscape resoluition cap, but ended up with a ton of waterbrushes and confusion, now I have only 1 per landscape and they function as intended.
It was a while ago, the memory is foggy, but not sure if you need to drop in new river splines after waterbrush deletion, I don’t think so, as another post has indicated removing the waterbrushes fixed thier river.
But then again, this may not be the problem… it could be an import bug ???
Another thing to try could be to use the Import button of the Landscape Mode, Manage section and reapply the Height Map over the existing landscape, rather than via the New button ???