I know this problem, it comes because the stream can not come near the capture device.
Stream is not being received by capture device that's the rainbow effect.
Problem is not the PI or Hyperion.NG but has to do with splitting of HDMI signals.
If the splitter hasn't got receiver ends active then you have this.
I used a splitter/downscaler from Xolorspace and never a problem. It has build in HDCP 2.2
I think your problem lies in EDID, the screen or grabber are not asking (the right) signal from the source, so only one becomes active trough splitter and sends the "rainbow"
Yes, I now installed a splitter into the mix, but the signal is still not going thru and I know that it should work.
try to use only the MAiN HDMI port and use the splitter there.
Can u make a schematics of your build?