IAP doesn't work in TestFlight on iOS 15

Hello, Apple and community.

Since the release of iOS 15 our team have serious issues with In-App Purchases in TestFlight builds:

• "Product Info" and "Restore Purchases" requests are never coming back. Once I've been waiting for more than 5 minutes and there were still no response from Apple. Nor error, nor success. 🤷‍♂️
• After TestFlight build of our app tries to use StoreKit at lease once, the device account, which is active in App Store, goes amok - all purchases are gone (even free apps/games) and even already installed apps stop working if they use account ID to validate premium features. Apple Arcade stops working as well. The only solution is to restart the device. After the restart everything is back to normal, until we launch our app again...
• Even the old TestFlight builds, which we released to App Store more than a months ago, have the same issues with IAP. Previously, those TestFlight builds were working just fine.
• App Store version works as expected, no issues at all.
• The issue only happens with our primary accounts which we've been using for app testing for 2+ years already. Also, the issue is gone if we log in with different (clean) Apple ID.
• We experience these issues in different geographical locations (Ukraine and Lithuania).
• My colleague even tried to reinstall iOS 15 (without restoring data from the backup) to no avail. The issue is still there.
• We currently develop 2 separate apps, both are using 99% the same IAP managing code. The TestFlight builds of our other app work just fine with our accounts, no issues at all.

Probably it's needless to say that this issue drives us crazy and makes developing our apps so much harder... 😕 Did anybody else experienced anything similar? Is there a solution (or any suggestions) we could try to get things back to normal?

Thank you. 🙏🏻