The issue we're talking about here is related to the web, not to the Spotify app. You create a website where you can login using your Spotify account (as described in the Authorization Code Flow docs). When the users clicks to log in with Spotify, they're redirected to the Spotify website, which authenticates them and then returns them to the original site (to the provided Return URI).

This works fine in every set up except one (at least that I'm aware of), which is when you go to the original website using Safari on an iPhone (up to date versions of Safari and iOS) *and* in that Safari browser you're not already logged in to Spotify. Then when you're redirected to the Spotify website you're asked to enter your credentials. And after doing so you should be authenticated and redirected to the original website. But instead, you're not redirected and just stay in the page posted in the screenshot above.

A video of this would not show anything except what I've described above and it will end in the screenshot posted above.

I hope this helps to reproduce the problem and look into it.