I use the Water plugin in Unreal Engine. I had lakes and rivers in my landscape — they worked and deformed perfectly. But then, at some point, the rivers simply stopped deforming the landscape. Creating new lakes deforms the landscape, but new rivers don’t. They continue to work on other landscapes though.
The landscape needs to be expanded to a square size. I had a 6x9 components landscape, so I expanded one row of 6 to a length of 9, and the rivers magically began to deform.