I heard from Fastmail staff. In the situation I described (message moved to Trash by a rule or plus+address target):
- If the message is from an address you have an identity for, it will be moved to your Sent folder. In this case, it appears that you are the sender (since it's sent from one of your identities), so the system moves it to Sent.
- Otherwise it will be moved to your Inbox.
So messages from others which are automatically filed in Trash should appear in your Inbox when undeleted.
Bill