I found a simple solution that worked for my iPhone 6s, ver 13.3.1.
The SENT folder was missing only on my primary DEFAULT email account which I'll call "A". (I have several email accounts setup in the mail app).
Solution:
- I moved a random message from the INBOX of account "A"to the JUNK folder (as suggested by in a previous post). But still no SENT folder in "A".
- Then under "settings>mail" I changed the DEFAULT account from "A" to another email address which I'll call "B" which was already in my phone. For some reason the SENT folder then showed up in "A" and included previously sent emails. Yay!
- I changed DEFAULT email account back to account "A" and all is well. So far at least. Luckily I didn't have to recreate any email accounts or lose any messages.
Summary: Change default email away from the address with the missing sent folder, then back again. And you may need to move a message to your junk folder as well. Not sure if that was necessary or not.
Maybe it will work for you. Hopefully Apple will fix this strange bug.