Sounds like HDCP. I've read that some cheap hdmi splitters will strip HDCP as an undocumented side effect. By strip I mean communicate to the player/source that the display/destination is compliant regardless of whether that's true.
I haven't heard that before, but according to Wikipedia a master key was leaked or reverse engineered 10 years ago, so presumably manufacturers out of Intel's legal reach can produce devices that strip HDCP.
(An alternate option might be to rip the 4K BluRays, which is doable if you have the right drive, but also a bit of a pain.)
This is what I have been doing with MakeMKV.