@Andrew-R Ok, decisive now: bug is real, but totally unrelated to either our mpv or altivec. Reproduceable with pure-C build of libvpx and ffplay.
P. S. Activate non-altivec libvpx (or rebuild with `libvpx -altivec), run `ffplay -loglevel verbose -vcodec libvpx-vp9 somefile.webp`, should throw out identical messages.