Not much detail here, but patch seems largely self explanatory:
Fixed in 0.33.1. Please bump.
"A format string vulnerability in mpv through 0.33.0 allows user-assisted remote attackers to achieve code execution via a crafted m3u playlist file."
GLSA request filed.
This issue was resolved and addressed in
GLSA 202107-46 at https://security.gentoo.org/glsa/202107-46
by GLSA coordinator John Helmert III (ajak).