Terrain stamping steps gone after saving

I’m kind of new to unity, so maybe I’m just dumb.

I created a new terrain and wanted to add some heights and depths to it by using the “Stamp Terrain” function. Created a few heights by using the brushes, adding a few depths… Decided to save and - poof - all gone. If I try to add heights again, the terrain seems to be a bit buggy and it create some ripples. Since it’s hard to explain, I took screenshots of before saving and after saving.

Before:

186400-bildschirmfoto-2021-09-18-um-174345.png

After:
186401-bildschirmfoto-2021-09-18-um-174406.png

This is what happens when I try to brush something after everything disappeared.