Using New Outlook on Windows for professional communication and productivity
The problem is - it looks like your IMAP account name is an email address from these screencaps. That works fine.
The issue is if you have an IMAP account name that doesn't match your email address.
Nothing you've posted has indicated that you're able to add accounts where the login and email don't match.
Specifically - this screengrab seems to show account logins that are email addresses. These are created via the Add Account settings in Outlook (new), which only accepts email addresses for user names, or are imported on the first run of Outlook (new) from Windows Mail.
This screengrab is the Outlook 365 control panel and isn't connected to Outlook (new).
Thank you for sharing your screengrabs - I appreciate it!! - but I think the problem still remains.