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.

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.