InApp Purchases testing always asks for payment methods

I want to test InApp purchases for our app. I followed the guide (ie upload the app to PlayStore, add products, add a testuser ourtestuser@gmail.com). We are currently testing on two different devices (one is on Android 6, one on Android 8), both of them have ourtestuser@gmail.com logged into the playstore. Both have PlayServices 19 and Playstore 17 installed. Both have the same (debug) version of the App installed via Android Studio. Both of them can connect to the billing service and query for available purchases. Nonetheless, the test-cards are only available on one of them.

Ie for device 1 (Android 6) when I start the purchase flow, I can select one of for available test cards (always approve, always decline, approve after a few minutes, decline after a few minutes) and buy the product, send the purchase to the backend and verify it with google play.

But for device 2 I get ask to specify a payment method (credit card, paypal, pay via mobile provider, redeem a voucher).

I tried to remove the google account from the device and add it again. No luck. I tried to remove the testaccount from the Developer Console, waited about an hour and re-added it. Then after a while, the test cards were available also on device 2. But as soon as I deployed a new version of my app via Android Studio, they were gone again.

I'm quite clueless right now, what to try next. Any hints are appreciated.