There is an updated pull request by ardadem against master on the void-packages repository

https://github.com/ardadem/void-packages pr-mpv
https://github.com/void-linux/void-packages/pull/25230

mpv: compile with uchardet
"If mpv was not compiled with uchardet, then utf-8 is the effective default."

non-utf8 subtitles are broken without this

ref: https://mpv.io/manual/stable/#subtitles

A patch file from https://github.com/void-linux/void-packages/pull/25230.patch is attached