Problem is HDR. Non-UHD Blu-Ray players need SDR so you need to tonemap. Methods are discussed in other parts of the forum, it's not a trivial topic:
https://forum.doom9.org/showthread.php?p=1827927
https://forum.doom9.org/showthread.php?t=174415
https://github.com/ifb/vapoursynth-tonemap
(That said, not all UHD Blu-Rays have to be HDR.)