I've been playing around with subtitles in general. Use ffprobe to see what you have, normally(sic.) :
#0:2 first subtitle (forced or not) say eng
#0:3 2nd say spa
#0:4 3rd say fre
So map subtitles based on -map s:0 for 1st which will pull #0:2, although -map 0:2 is fine.
People forget video/audio/subtitles etc are all ...