How to fix that?

Is it AA?

Looks like aliasing. The reason it is flickering is probably due to the temporal anti-aliasing.

Easy way to test is to just turn temporal antialiasing off. If the flickering stops, it’s probably TAA.

As for solutions, if it is indeed TAA then you can try switching to forward rendering and using MSAA instead. Otherwise I would suggest building an LOD that replaces the geometry with a masked texture plane at a distance, the mipmaps should help solve it.

looks like a temporal artifact. try using fxaa or msaa