I have an email account configured via IMAP in iOS Mail app, with the Organize by Thread and Complete Threads toggles both turned on in Mail settings.
Thanks to the latter, when I open an email from the Inbox or Archive mailboxes, it is shown in the context of its thread, with the preceding and following emails also being shown above and below the opened one. However, when I open an email from the Sent mailbox, the thread view does not activate, and only that singular email is displayed.
Is there a technical reason why Sent is being treated differently from Inbox and Archive in regards to threading? Is it possible to enable the thread view for emails opened from Sent, as well?
From my experience, this behavior persists both on iOS 13 and 14. (Perhaps it dates all the way back to the introduction of the thread view in Mail?) And, as far as I can tell, it does not seem to be specific to the email provider.
Unlike iOS, in Mail.app on macOS, viewing an email from the Sent mailbox does show the full thread it belongs to.