I’m using the Facebook app for years, but most of the time, it still changes the quality of my uploads. Though HD is enabled, uploading a photo in an excellent quality seems to require luck.
As a workaround, I upload pictures through the Facebook website.
I noticed that it uploads the original file rather than compresses it.
Here’s an example of my recent post through the mobile site.
There are limitations like re-arranging the photos, so I do it instead on my computer.
Basically, I upload photos on the website, then I use my phone or PC for editing. If I will re-arrange the photos or whatever, I set the post’s visibility to “Only Me”, then change it to “Public” when ready.
P.S. Tested on iPhone.