Why does opening an email from Sent mailbox not show the complete thread it belongs to in iOS Mail app?
You are correct, the sent mailbox behaves differently than all the others in my testing with iCloud and O365 and Fastmail IMAP.
I wouldn’t presume to speak for Apple when they decided how to implement those switches, but their intent is very clear and easy to reproduce. Perhaps someone familiar with IMAP standards or other mail clients can see if this is typical for sent to be strictly ordered by time or that was just one team’s decision when they coded the app.
I see no configuration profile or setting to make sent work like inbox with respect to threaded treatment of mails.