mpv - subsitles not rendered per mpv.conf even with --sub-ass-override=strip --embeddedfonts=no

I got a mkv file recently where I noted mpv renders subtitles not as I put in mpv.conf. The file has two subtitles streams and one is displayed as describes in mpv.conf, but other is just same no matter what I do: press shortcuts to move, change fond size. I thought it was embedded in the video, but no, switching subtitles turns in off. Web search gave https://www.reddit.com/r/mpv/comments/e2ixcd/how_do_i_override_ssaass_font_colors/ , I've tried it, read man page, then tried to run

mpv --sub-ass-override=strip --embeddedfonts=no acme.mpv

those subtitles were just same as before (I was able to change "changeable" ones with CLI option). What else to try? **Edit**: ffmpeg shows that the problematic stream is ~1000x larger in size than other subtitles stream, which hints me it is not text, but video type... then solution is to find other stream. How to test if my guess is correct?