Some input about the issue how I solved (not sure it solves the problem for others):
First I created own blueprint based on Unreals’s River Body BP and I switched off “Affects Landscape” by default and I added this version to the map to avoid breaking World Partition. (Some defaults like river materials need to setup again, seems it not initialized correctly if I use this child class)
Second important thing I also disabled “Affect Height Map” in the Water Brush Manager.
Now my rivers are loading/unloading with tile and no more full landscape loaded issue.
Of course it means landscape will not affected, but it wasnt an issue for me as I’m working with presculpted river beds.
P.S. found a topic about it, it adds you need also delete Water Brush Manager which will be regenerated next editor start.