IOS16 Mail app eating sent emails

Hi,

Upgraded my iPhone 13 Pro to IOS 16 and now on random occasions I have found sent emails via an on-prem MS exchange server using Mail are being lost. Not in the sent items or draft or deleted items. I can see there is now undo send at the bottom of the app but definitely avoiding touching that area once the email has been sent.

Frustrating as now cant trust it anymore to send emails to recipients. :/