I have the same TV. It doesn’t specifically help you but using HDMI 2.1 sources works fine for 4K HDR 120Hz and VRR. I have too many devices (Xbox Series, PS5, AppleTV, Switch) so they are routed through a GUIDE3 switcher.
So I would think it is the DP converter or the HDMI cable not really being 2.1.
I have the same TV. It doesn’t specifically help you but using HDMI 2.1 sources works fine for 4K HDR 120Hz and VRR. I have too many devices (Xbox Series, PS5, AppleTV, Switch) so they are routed through a GUIDE3 switcher.
So I would think it is the DP converter or the HDMI cable not really being 2.1.