I do not remember which was the first version that worked fine. For some time, "gpu-next" was not included by default in the releases, but you had to compile a custom variant to include it.
Pay attention that "libplacebo" is required. If libplacebo is absent, then the Dolby Vision conversion will not work. This could be the reason of your problem, if your Linux distribution does not have libplacebo as an automatic dependence of mpv, or if mpv has been compiled without support for libplacebo.
I would try updating to 0.37.0 or 0.38.0, IIRC it only started to work for me after I updated to one of those (I always build mpv from the latest commit myself, so I'm not sure which release version exactly has the crucial fixes).
