I just had this case where the Calendar App Owner left the company a few months ago, and I had to transfer ownership.
Before I could transfer the app's ownership, I had to locate the source sheet and share it with our System Admin account.
As both a Plan Asset Admin and a System Admin, we must have a notification or an easier way to track premium apps created by deactivated users. This would enable us to promptly transfer ownership to the appropriate user, a capability we currently lack.