Yeah isnt a bug, its because your block are locked.

Just right click on the block in the Block editor and "Unlock block"

Its worked for me so tell me if this tips solve your problem :)

Cya !

It's working but when I build my map the "bug" reappear. Thanks for your answer :)

Seems like a texture material problem.
Did you edited the terrain texture or added
New texture with the terrain tool ?

Try to replace the texture with the terrain paint tool then build map
Again.

If it ´s not working , try to delete this one terrain
Block then , add a new one and sculpt it again.

Maybe i ´m wrong , so, make a map backup
Before to tweak the tools.
And if you break something, you have some
Backup.

Thanks for your help but it doesn't work. When I build the map the problem reappear...
And I've no custom assets or textures

Try to use Validate and Build -> Build Assets for PC.

Unfortunately, it doesn't work.

"Edit Terrain" > "Material Painting" > "Regenerate Layers". Then try what Rafał said.

Thanks, but it doesn't work.