I've noticed that if I pass the --config=yes option for mpv in SMPlayer's advanced preferences, it will use some of my mpv config, since it'll load the autoload script I use, but it'll ignore all the settings needed to load YouTube subtitles. Setting disable_player_config=false doesn't change this.
Celluloid, meanwhile, will load subtitles, but using it feels kind of jank compared to mpv by itself. On my setup, you have to pause while it's "loading", then play to actually get a YouTube video to start, and you also have to press J to bring up the subtitles.