Hello Matthias,
Thank you for finding time and try the troubleshooting.
Based on your reply this issue applies to all iframe embedding and SharePoint sites in your tenant. In this scenario this behavior might require further investigation from backend we would suggest you create a support ticket on the issue from Microsoft admin center > Support. Agents on server side have more resource and tools to troubleshoot the issue further and find an answer to this concern.
Thank you for your patience and understanding.
Best Regards,
Igor