Stream videos aren’t deleted when a user who was the original owner/recorder/uploader of those videos leaves. This is because after record/upload the tenant owns the videos unless an owner explicitly deletes them!
Hmm. Is there an internal support desk or internal support team which manages your Office 365 tenant? They would be the people to direct too. Obviously, Microsoft is a pretty large organisation so I couldn’t say which Team that actually was. However they would need to be Stream Admin or Global Admin to be able to do it.
Maybe worth reaching out your end to Mark Mroz or Vishal Sood your end who are right up there in the Stream Team who may know who this is.