I am seeing the same thing on iPhone 12 Pro Max iOS 16.2, would be great to get an update on this issue.
CharlieHorton
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As a work around, I was able to create a new sandbox user by going to App Store Connect -> Users and Access -> Sandbox Testers, I signed out of my current account and logged in with the new one and I can view the manage account page.
I did this on an iPhone 12 Pro Max running iOS 16.2