Why Testflight In-App Purchase cannot get finished?

Question

• I have auto-renew subscription with remote validation set up.
• Everything works perfectly when testing IAP with plugged device and running through Xcode. Sandbox is my friend.
• But when I upload to Testflight, and wanted to execute the payment (ie subscribe to monthly/yearly plan) apps acts like you would expect, you confirm the purchase then it shows alert that "Everything is set up and you are ready to roll" but it get stuck at this "finishing" when big activity indicator is running for EVER. Here Sandbox is not my friend.
• When I dismiss the app open again and try to subscribe it shows that I am already subscribed, but it seems it is not sort of "finished" and dismissing logic is not triggered (like you would get from purchase page redirected to Home page or whatever page you want)

Interesting is that we have the same way of IAP (for auto-renew subs as well) at other app where it does the same issue, but it got approved through the review and we normally notice purchases in production.

But the Testflight does not simply work. I did try all "the magic" with different Sandbox accounts and none of them worked. Any insights?

I also read that it can be Sandbox environment bug, but how to resolve that or at least how communicate such a bug to my manager who has zero development experience but want to test the purchase in Sandbox environment.