Oh yeah, maybe the water system is struggling to calculate.
I have noticed when a few water systems in an area it causes issues.
When one waterbody is above another, it can causes the one above to not render, but usually noticible in play mode, can get around this by making smaller sections of water instead of a bigger one, but I haven’t tried it with river systems, maybe offset the above and below nodes a bit further apart.
There seem to be Spline sweetspots and sourspots, node positions will affect other waterbodies
Like the ocean, position of the ocean splines can have a big impact on all waters
Have you thought about turning off affects landscape for some bits and manually sculpting the terrain ?