@Manuel Plath Thanks for posting in our Q&A.
Based on my search, I find an article about how to disconnect Managed Google Play Account in Intune. Please refer to the steps.
https://www.azure365pro.com/disconnect-managed-google-play-account-in-endpoint-manager/
Note: Non-Microsoft link, just for the reference.
If all the steps are finished, but this issue still exists, it is suggested to create an online support ticket to find if there is any method can delete Managed Google Play Account from backend. Here is the support link:
https://docs.microsoft.com/en-us/mem/get-support
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