There is an issue with iOS embedded captive portal assistant in case the user is using 2FA on Facebook.
User enters Login/pass on Facebook
FB asks for 2 factor authentication (SMS sent or code sent to FB app)
User switches from pop-up to app or SMS
User looses the pop-up, and cannot enter code
Unfortunately this is how the OS works. If the user closes the popup, he/she needs to start the process all over. One possible solution is to enable the sticker notification on iOS (by the user) so they don't have to close the splash popup.
But we cannot change this behavior as this is out of our scope. For these users who have problems, you can offer the skip social feature with an alternative login method.