The audio plays fine, and the window comes up the right size, but it's always black, with a variety of files (mp4, mkv, m4v, avi, swf, at least; what I had handy to try).
It worked back on High Sierra (same version of mpv, I think). And I made sure I built ffmpeg with the same variants I'd had before, so that shouldn't be the problem. 0.29.1, which I probably downloaded from the mpv.io website, is working fine. The library versions shown when using the --version option are identical for the MacPorts build and the downloaded build. Do we need to switch to the current version?
