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 !