Disappearing emails
I am using Mail Version 14.0 (3654.120.0.1.13) on BigSur 11.6 (Mac mini M1).
Recently, I had the same problem twice: I receive an email with some attachments; I move the email to another folder (once to a local folder, once back to inbox from the spam folder by clicking on the corresponding button on top of the email). Then my email fully disappears when closed. A search in Mail in all the accounts does not give anything.
In both cases, I had to figure out how to contact the sender to ask for a copy of the email.
Mac mini, macOS 11.6