This issue bothered me for a long time: whenever I uploaded my video to Instagram, the stereo sound of my video was converted to mono.
Then I read a comment on a blogpost that suggested a simple workaround.
Instead of using an Apple device for your upload just use a Samsung device for your upload.
Now your Instagram video remains in stereo
I don´t know if this issue is of importance to you. It was a big deal to me though. That´s why I´ve decided to share it with you guys.