This isn't an issue with Samsung phones or something Samsung can fix. This is a problem with Instagram not working on all phones with an 18.5:9 ratio (I don't know about iPhones aspect ratio and they use a different software anyway, so dunno how it works there), and Instagram doing nothing to fix it. This has been a problem for a long time now, and until Instagram fixes it there is nothing anybody else can do.
Don't hate on Samsung, or android, hate on Instagram for not caring about their android users.
Why should Samsung go in and try to reverse engineer a single app when Instagram just needs to add black bars on the edges of their videos? You definitely wouldn't see apple try to do that, they only accept or reject apps in their app store.