blue "unread mail" dot just vanishes, without being read

if it's an imap or exchange mail setup then if it was read on any device that had the account added it would be read on all

same if it was read on a webclient.

if you do not have added the account to other devices which could have marked them as read then I would consider changing the password for the mail account

I face the same issue on my Iphone XR IOS 14.3.

The setup was automated using known parameters for icloud setting the app has for this exchange.

Checked that all ports and security settings match what is recommended here: https://support.apple.com/en-us/HT202304

Receives all email and send email without any errors.

Just cannot get the APP to retain the Unread Count on the APP icon.