For years it has been a mystery to me why I cannot find some emails I've sent to the trash. Even after only minutes. Just now I have discovered that it is because Gmail has assigned the to a category. Apparently when an email with a category is trashed it does not go into the trash as one would expect. Instead it goes or remains in the category. This is counter intuitive and a poor user interface design. Trash should mean trash not just removed from the Inbox. I personally have my own filters and don't use categories but thought them harmless. Not I see they are not.
Either allow me to turn off the categorization OR change the Trash action to always put the email into the trash.