I also ran into this issue, and it seems that I have disabled Facebook from my Instagram account in my account settings. But it wasn't that easy, and some of my posts were still being posted on Facebook. So I just switched to Snapchat because it's basically the same content as Instagram, plus you can upload videos to your phone, and there are no annoying ads. Well, another reason I switched to Snapchat is that I couldn't
block someone on Instagram. For example, those who send out spam mailings or writes inappropriate words, thereby polluting my messages.