lpisaniello
I tried to replicate at my end but no luck.
I unpublished / trashed K2 item from an author and when there is no published item from an author, the author even not show up in the filter list for author and the same for trashed item.
You can share your site info so I can check directly for you or share me a backup of your site so I will check in my local. It's hard to help you out without seeing the issue.