Anyway I tried with my other HDMI 18gbs cables which work a treat from the 4K server straight to the PJ and I still get artifacts through the HD621 so I can confirm that at least my HD621 will not pass 4K (UHD) without artifacts. This is via the Oppo upscaling the blu rays to 4K and sending it through the HD621 as since the HD621 cannot process HDCP 2.2 I cannot use the 4K server through it without using that other solution you provided.

I thought I was running the latest firmware in the HD621 as I did purchase it from Rick in the UK last year.

Theatre Room: Sony 4K VW500ES, 861v8+ID41, HD621, DSP6000's, DSP5000C, DSP5000's, Oppo 205, Sat STB, XBox One X, Stewart Cinemascope Screen, QNAP TVS 1271U-RP (96TB) with Sooloos/Roon core pkg.