Hi @Markus Männl ,
As per my knowledge, deleting users' account and profile will not remove personal site immediately. It will be deleted in 30 days by default and then the deleted personal site will be moved to site collection recycle bin. The default retention period for OneDrive is 30 days, but you can change this to longer or shorter. So, check the retention period on your tenant and if It has been more than that time since the user account was deleted. Reference: The OneDrive deletion process.
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