P.S. I have contacted Apple support by telephone about this a couple of times, but they were no help at all. Once a support employee suggested turning off notifiactions for all of my mail accounts (which effectively removed them all). The last one I spoke to simply hung up on me. I have reported this problem to Apple Feedback but, of course, Apple provides no feedback.
There are several threads here with the same complaint, e.g.
"You Sent this Today" Follow-Up - Apple Community
Turn off new follow up function in IOS ma… - Apple Community
Sent mails showing in iCloud Mail Inbox, … - Apple Community
What is the "follow up" I am getting on s… - Apple Community
Mail follow up suggestions - Apple Community
Follow Up feature auto-sent email without… - Apple Community
iPadOS is now on version 16.2, after several minor and a couple of major updates, but the problem persists.