TriColors

Found the culpirt 2mins after I posted that, it's the Windows 10 scaler that tricks Speccy, if you set it back to 100% it becomes 4k again. The 150% reports that resolution

I have this exact same problem, even though my second 1080p monitor reports correct results. The common factor here is this ASUS monitor itself, I own the exact same one I couldn't find any firmware update for it though.