Video clean feed

I have a 2nd monitor Benq pd2700u connected to 27inc imac. running resolve 17 .
The 'Video clean feed' on the Benq is terrible quality. Lots of artifacts in the shadows. It looks like a poor quality low bit rate video or heavily compressed?
When I use 'Dual screen' and move my interface viewer to the Benq, it looks great. Clean image just like the imac.
Dual screen on FCPX alos looks great on benq.
So monitor is set up ok? and connected by usb to display port.
It must be a setting causing poor quality on the ' Video clean feed' /
The 'Video clean feed' on the Benq is terrible quality. Lots of artifacts in the shadows. It looks like a poor quality low bit rate video or heavily compressed?
When I use 'Dual screen' and move my interface viewer to the Benq, it looks great. Clean image just like the imac.
Dual screen on FCPX alos looks great on benq.
So monitor is set up ok? and connected by usb to display port.
It must be a setting causing poor quality on the ' Video clean feed' /