Hi guys,
I found a solution:
All the sandbox users that I had setup at https://appstoreconnect.apple.com/access/testers had another App Store country/region setting (not USA). Then I created a new sandbox tester user with App Store Country/Region setting to "United States".
Also I did NOT click the verification link from the verification email that you receive after adding a new tester
I signed out from the previous sandbox tester account on my iPhone (Settings > App Store > Sandbox Account > Sign out)
Then I started the app again that I wanted to test, I started the in-app purchase, signed in using the new sandbox tester data and was able to complete the in-app purchase without receiving the "Terms and Conditions" prompt.
Btw:
I was not testing using TestFlight. I created an IPA-File in Xcode and airdropped it to my Phone.