Subscription never renews on Sandbox

Hi,
We're experiencing an issue where the subscription never renews on Sandbox no matter what we try.

There is always 1 in app purchase (the original purchase) in the purchase receipt. To confirm we didn't miss a renewal in the transaction observer we "restore purchases" and check the receipt again. Nothing changes, only the original in-app purchase can be found in the receipt.
Only a brand new subscription purchase adds additional in-apps to the receipt - we never see any renewals.
We have created new test accounts to confirm that the issue is not related to the auto-renew limit. How are auto-renewals meant to work on the Sandbox?
Any ideas?
Thank you

Subscription never renews on Sandbox
 
 
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