Flattening of water bodies

Hello, and Happy New Year 2025

In my project, I would like the surfaces of the water bodies to be flattened over a terraforming heightmap (terrrain mesh).
Some water bodies, which are missing in the default scenario, I can easily transfer them into the scenario and in Dev Mode I can also see such water surfaces perfectly leveled. After having compiled, inside Dev Mode, a .bgl file without errors, upon exit the changes are lost.
So, I assume that some parameter is missing, my question is which one?
I can share a copy of the project if it is useful to understand the problem.

Regards