When an email is moved to the K-9 trash folder by clicking on the trash can icon, is it automatically deleted from the server as well? If not, at what point is it deleted from the server so it won’t show up on other IMAP devices?
If your K-9 account’s trash folder is linked to the server’s trash folder, it should move there once deleted on K-9.
Deletion from the server will occur depending on usual configurations there. Sometimes 30, or 90 days.
You may be able to delete it from trash to permanently delete it. Best that you try some examples out to see how that works with your server.