Sent emails vanished from iPad Mail

This is an iPad 8th gen running 18.7.1. All its send emails suddenly vanished a few days ago. Checking on an old iPad, they were there for a few seconds after switch on but immediately vanished as we looked when Mail was activated. Checking on the web, everything else is there but no sent emails. The account used is the Apple iCloud account. I have tried a hard restart. Can they be recovered? And how do I prevent all sent emails from vanishing in future?