When I reach Instagram user pages while in Incognito mode, Chrome pops up a window with my non-incognito username asking whether or not to sign in. See screenshot below. Although I assume Chrome Incognito isn't broken and sharing this information with Instagram - I do wonder how Instagram caused this dialog to show. It is a bit intrusive and I would have preferred if Chrome didn't do this. I've also never seen it anywhere on the internet, so it is a bit of a head scratcher. How'd they do it?
Instagram can't, and isn't, doing that. Chrome is. Chrome knows your Google account because you've signed in to it, and they know that you have your Instagram credentials saved in your Google account, so when you visit the Instagram website, they offer to sign in automatically for you.
I don't know why this would trigger in Incognito mode; that seems like a possible bug. This is by design, apparently. I find this a bit worrying.
This popup isn't disableable as far as I can tell. If you're getting it, it means you've disabled auto sign-in and require a confirmation; that's as good as Chrome will give you, unless I'm missing an additional setting.