Currently, if you're running Big Sur and if you have a POP mail account, deleted mail does NOT go to the Trash folder in Mail on your Mac. It's just deleted locally. However, if you access the POP server via the WEB you'll see it there in the Trash folder on the server (if you haven't chosen otherwise).
With IMAP, the server Trash folder is echoed on your Mac, which is why you now see your Trashed email.