That sucks, for the first time I was able to get my TV to recognize 23p as 24hz if I use CVT CRT timings.
But giving up on 12bpc it's not a good trade off.
One idea would be actually change the timing for 23hz based on the findings with MadVR and put those on CRU (custom resolution utility).
But I don't know how to modify timings of specific hertz like 3840x2160.
EDIT: Got 12bpc to stick after a reboot.
Used CRU with the timings I wanted, added as a detailed resolution, moved it up to be together 3840x2160@60 and after rebooting I selected 23hz, changed to 12bpc and the 24hz (23.976) I had as 12bpc.
Rebooted once more, changed resolutions and still 12bpc.
So, yay!