Here's another Example of World Gen
Best Guess, this happens when Mods are Updated or in Conflict as the Terrain Generates
This is with projectLUMA 1.32, Medium Notice the Flowing Water Transition and Flowing to Still. The Still and Transitions look like Shadows. Just an FYI if you run Shaders
This is with BSL 7.2 and Water Color Set to "Vanilla" a Nice Blend where the Flowing Water Meets the Still Water
The Beauty of the Streams Mod adding to the Environment. Stream is on the Right