Hello!

Does anybody know why my render view colors are completely different from scene view (render region tool)?

I'm using arnold with ACEScg and rec709 color profile.

Okay, I managed to solve this issue.
What you need to do - enable correction toolbar in both render view and scene view and set it to ACES / rec709 or sRGB.
Then open display options in the scene view - scene tab.
Set gamma to 1 and put there sRGB to ACEScg or rec709 to ACEScg lut (or simply put 2.2 value there) and it works.]

UDP: Actually if you type anything in the LUT field - it works, any number or even "hello world" solves the issue.
Probably a bug. Just make sure to set gamma to 1.0