I would like to reopen this issue as it has not been fixed by Instagram till to this date. Reporting the bug doesn't seem to get Instagram's attention and I didn't buy a $840 phone just to have unoptimized apps claiming to be optimized for infinity display.
THIS PROBLEM IS NOT SOLVED AS OF 5/17/2018 AND SHOULD NOT BE MARKED AS SOLVE.
Have this problem on my S9 with Instagram. It's forced to be in full screen mode but cuts off the sides of the pics. Super frustrating that the Samsung software does not let me take it out of full-screen mode. This issue has definitely not been fixed as of today.
THIS IS STILL NOT FIXED!!! Samsung, I beg you, work with Instagram or whatever and find a fix!!! I'm managing 3+ social media accounts on my phone, and when the creative does not match the screen it gets QUITE ANNOYING. You gotta understand this is actually literally my job, so please please fix it, or suggest a workaround!!!