I've spent hours looking into this. All info out there is outdated and no longer applies. Facebook claims I need to go to my "Apps and website" section and find Instagram and edit the audience there but Instagram doesn't even show up to begin with, despite me relinking via the Instagram app multiple times.
Nothing I do is allowing me to change the default audience for a post shared from Instagram. This is incredibly frustrating. Does anyone have a solution?
Related: https://www.facebook.com/help/community/question/?id=10217331234301292