Sorry about the late response! I couldn’t replicate the issue even after a couple of days of trying. I got some visibility issues for one of my test maps, but after unloading and reloading them with WP it corrected itself.
I made 2 test scenes in 2 different versions. 4.27 and 5.03. Both were using landscape and water systems, one had 2 different landscape scales (like your scene) and the other was just water and an island.
I still believe this is a migration up to 5.1 bug, but I can’t replicate it at all. I’ll be keeping tabs on it, but in the meantime it’s a good idea to submit a bug report with as much information as possible!