I don't know where you're looking but it's very possible to get multiple subtitle tracks. The problem however is when there are multiple variants of a language.
Eg:
En-US
en-GB
nl-NL
nl-BE
es-ES
es-MX
One language then overwrites another leaving you with 1 track instead of 2. That's a bug the staff is aware of and will hopefully issue a fix for soon.
Sent from my Pixel 3 XL using Tapatalk
"define" include? If you use the "embedded" option, each subtitle track will get added into the mp4. You select 5 subtitle tracks, you get 5 srt files or 5 tracks in the mp4