Water Feature still buggy in 4.27?? (I need an adult!)

Urgh. After playing around with the water system in UE 5+ and learning that it absolutely does NOT work with an imported heightmap, I reverted back to 4.27 where I had some success. However, as I try to add a simple lake I am getting some really weird things happening.

First, although when I first added a lake it was fine - now when I attempt to add a lake no water appears regardless of which water material I use.

Secondly, as you can see in the video, it affects my landscape in a really weird way; my poor mountain in the background is affected for some reason.

Admittedly, I am still learning how to utilize the water system but I am pretty sure this is NOT how it is supposed to function. Is it just a bug with imported heightmaps in general or is my landscape Z position somehow causing this? I’ve tried setting my Z to 0, 100, 1000, 5000, -100, -1000 and it doesn’t seem to change the outcome.