Closed issue, problem is gone in (not specifically) Godot 4.0 beta 16.

The issue still persists, but only on NVIDIA cards. Since I opened this issue I have switched to AMD, so this should have been closed.

I can confirm this on 4.0.rc 493e932 (Linux, GeForce RTX 4090 with NVIDIA 525.85.05):

No MSAA	2× MSAA	2× MSAA (second try)	4× MSAA	8× MSAA

MSAA artifacts on the right side aren't always identical – I tried to go back to disabling MSAA and setting 2× MSAA again, and the artifacts that appeared on the right were different.

In comparison, enabling TAA, FXAA and debanding doesn't have any effect on the normal buffer (as expected).

Got same problem on Godot 4.0.2.stable.official 7a0977c
(Windows 10 22H2 19045.2728, GTX 1660 SUPER with driver 527.56)

edit: seems the data in normal buffer is actually correct, maybe it's just something went wrong when editor tries to draw it.
I tried to use RenderDoc, I saw a correct normal texture in some buffer.
Also, I have an outline shader which performs sobel operator on normal texture, it's working flawlessly