One of my screen is flickering but it's not monitor issue

Hi I recently got a problem I can't solve and it drive me nuts. Something like 2 weeks ago one of my monitor start flickering and dimming aggressively. I wasn't sure what's going on but after all gpu driver reinstall helped. But it starts happening today again and reinstalling driver is not solving it.

I have 2 different monitors, both plugged with display port. One is working fine but it's old 1080p 60hz monitor and my main one is 1440p, ultrawide curved 165hz monitor. But it's not monitor problem because I checked with laptop and console and it works perfectly fine, it's only giving issues when plugged to my PC. I already played with all the settings and nothing is working. Hdr on, off, auto hdr on, off, hdr on monitor itself, free sync, override, auto dimming, I checked all on and off, nothing helped. I changed refresh rate, power settings, it was always ultimate but I still checked and played with advanced settings. Played with Nvidia settings, changing brightness, RTX hdr on/off.

Checked this monitor with DP, with HDMI, different DP cable, still same.

If monitor works perfectly fine with other sources it must be software? But why it affects only one monitor if both are plugged and are in extended mode? But that's also make no sense because it affect BIOS too, it's not flickering but it's dimmed, but steady. But why laptop or console is perfectly fine?

What also I noticed, I can't turn off my second monitor. If I turn it off there is window disconnect sound like for mouse or keyboard and after few seconds it's turning it on with sound of plugging new device to Windows.

But flickering on main monitor happens even if second monitor is fully unplugged so I don't see connection here. But it only happens when flickering on main monitor happens cause it was like that 2 weeks ago when it happened for the first time. Other times it works as it should, it just turned off without any sound.

I'm usually tech savvy, I'm fixing all Tech stuff and different appliances, building PCs etc but this one fully defeated me and idk what to do.

I also turned off mpo in registry I don't have time right now to fully reinstall windows and all apps, games, export and import settings etc.