As reported by Alfonso Crawford:

After some tooling around, I found a potential bug. On line 113 of
ytmpc, vidurl was being set in a way that "--max-quality" as appended
onto the front of whatever parameters I specified. What ought to have
been "-f 5 --max-quality=34" was being passed as "--max-quality=-f 5
--max-quality=34", which invalidated the customization. To fix the
problem, I changed line 113 to the following:

-> vidurl="$(youtube-dl --max-quality="$YTQUAL" -g -- "$id")"
-> vidurl="$(youtube-dl $YTQUAL -g -- "$id")"