We have recently updated several test devices from iOS 14.7.1 to 15.2. After that, when trying to make a test purchase inside our app, TestFlight ignores the sandbox accounts (which we add to Settings -> Apps-Store -> Sandbox account) and uses the main account that is used for purchases in the App Store.
The SandBox account works fine if you log out as the main user in the App Store - but then you can't use TestFlight to download test builds.
maybe someone has faced a similar problem and knows the solution or will tell me in which direction to look?
TestFlight ignores the sandbox accounts (which we add to Settings -> Apps-Store -> Sandbox account) and uses the main account
That is correct behavior. The sandbox account is for when you are running an Xcode build. TestFlight, on the other hand, always uses your real account, but it doesn't charge you (and the same for your beta testers).