Okay, I won't use the test map again in the future.
I solved my problem. I cut the map into a few parts that I removed one by one until the time to identify the problematic places. I confirm that the problem is related to the heightmap. Somes mountains had to be completely removed and impossible to add new ones to these places without making the bug reappear. It's a bit annoying but at least I'll stop struggling on that xD
Thank you all for your help !
Thanks for your answers,
I will try to export / import the map later.
When I do my tests, I directly use the "test map" button on the editor. Are mods installed from the FAF client activated when I play from the base game?
I only have 2 UI mods active simultaneously, Nvidia fix and Philip Crofts soundtrack or Total Annihilation Music.
@jip
Yes the bug is only present with certain positioning of the camera on the map as well as a certain degree of zoom.
I think the bug is water related. when I remove it from the whole map the bug disappears
I have a friend who is also trying to create his water map and he has the same problems as me
Edit: just to be sure, the map editor names it: fafmapeditor_alpha_v0703?
@jip I haven't changed anything on the light yet. here is a screen :
@jip here is a screenshot where I set a height of 40 maximum on the entire map. on this version the craters in the angles are not present. (I made several versions so as not to break my map during the manipulations) 
Hello,
I'm trying to create my first water map.
When I want to test it, I have a issue. the bottom of my screen is bugged.
I read on the wiki that it is the "black plane" bug. I reduced the height difference between the highest and lowest point so that it is less than 50 (well I hope I did it right, the map editor shows me no more alert message about this when saving) but the problem is still there.
I then tried to raise the map in order to create a deep crater on a corner as it was indicated on the wiki, but still the same result.
since then I have been desperately looking for a solution for hours. I'm trying to figure out what I did wrong, does anyone have any ideas to help me fix this?
Thanks