it's a nightmare, latest iphone 11, latest ios 13.3 with the oldest app "email" doesn't work, email sent folder is not working
sent emails doesn't appear , may be you send one and appear on a global sent email folder, but await a while or just switch off-on you will never will see again the sent folder, by the way the sent folder of each email account missing forever.
it's sad that apple support can only request to customer manually delete-create email account but issues remain, you will see next day that sent folder disappear
I have this issue from the time I change of mobile to Iphone 11& IOS , I think support must be worried to sell iphones that give a good service, three month awaiting an update that solve this and nothing.