Going to post here so I don't spam your comments page:

River generation seems bugged when using this with Map Preview. Switching between River Sizes doesn't show the changes in Map Preview. If you change Map Preview's Optimize River Gen back and forth, the changes can be forced and the initial setting doesn't matter (River gen breaks with Map Preview's Optimize setting off or on).

It's odd, on some squares it will work perfectly and others it won't. I think it might have to do with the River's position, direction or size but I can't figure out the cause. Zero errors in log.