Years later, I see, and this is still an issue. I'm a developer in Germany and I'm trying to get stuff set up in App Store Connect but I've no idea how to handle the United States tax form section.

I'm having exactly the same issue when using TestFlight and the sandbox environment. One minute the pop-up appears almost immediately while the next, it may take 5 minutes or even not pop up on my device until later, when the app isn't even in the foreground. Am I right to think that this is just sandbox behaviour? I've heard mixed things.