As an addendum, delaying the default subtitles (dec/inc_sub_delay) also doesn't work before selecting the subtitle track in the menu.
Strangely the A-V delay (dec/inc_audio_delay) DOES work.
The problem seems to be that initially, the player uses the correct tracks, but somehow doesn't mark the tracks in use as active. Previously the current track would be indicated with bullet point/circle icon in front of it in the menu. This only happens after selecting one in the menu now.