Using classic Outlook for Windows in business environments
If this problem still can't be solved, please tell me if your e-mail is IMAP, POP3 or Microsoft Exchange account
Incidentally, I've only ever seen this issue with non-Exchange mail accounts. I have two IMAP accounts (one of them Gmail) with my Outlook (Version 2210 Build 16.0.15726.20188) 32-bit.
As a Snr IT Professional with Windows application development experience, it seems to me that whenever an open POP3/IMAP email undergoes a change e.g. when you click on 'Download pictures' or 'Add to Safe senders' then it's rendered as an unsaved message. Outlook then seems to get confused about saving the "changed" message back to the mail file, when it really ought to be a simple process!
@yuhao - you won't be able to reproduce the issue if it's an Exchange/Exchange Online account.
I recommend that you try setting up an IMAP account (Gmail, Hotmail etc) and you will be in a better position to see the problem :-)