Same here, i do get some posterization with my jvc rs440 when i do a p3 3dlut since it covers 92% p3, with the lk970 i get worse posterization for rec709 3dlut. But with the M150 i dont get any at all when i do bt2020 3dlut while it covers around 90%, i guess there is other factors to it, maybe color luminance .Well I get the same posterization issues if I for example try to make a P3 3DLUT for my JVC NX5 even though it's like 94% coverage.
I also saw it pretty bad when I made a rec709 3DLUT on a friends NX7 when I was making it over a stock calibration which was only 89% rec709 coverage (before autocal).
I am not saying it's because of not reaching the full gamut, but I just don't know what causes it to be honest.
I can make a P3 3DLUT for my LCD PC monitor which is only 78% coverage (it's not even meant for P3 or HDR) and there are no posterization artifacts.
So I really don't know what causes it, but I have seen it often on various projector 3DLUTs.
I wont say that every projector has it, but so far most that I have tried seem to. The only time I haven't seen it on a projector was when the projector could reach or exceed the target gamut. So I feel gamut coverage is somehow tied in there, though it's definitely not the entire story by any means.
In Lightspace there is a color gradient test image that shows all the artifacts pretty clearly if theres any after the 3dlut, and its extremely smooth and artifact free with the m150.
U may be right that no content probably get over 90% bt2020 to show any error any way but if that is the case why does ur monitor doesnt show any.