Miscellaneous topics that do not fit into specific categories.
Thank you for the additional details. Since the messages are visible in another email app and on the server, but New Outlook shows certain folders as empty or with "0 items," this appears to be a synchronization or local cache issue within Outlook rather than a problem with your AT&T/Yahoo mailbox.
The fact that some folders eventually repopulate on their own further points to Outlook not consistently syncing all IMAP folders.
In the past, a useful troubleshooting step would have been to compare the behavior with Legacy Outlook. However, Microsoft has been disabling access to Legacy Outlook for many users as part of the transition to New Outlook, so that may no longer be available as an option.
Have you tried removing the AT&T/Yahoo account from Outlook and then adding it back? Since Outlook required you to re-enter your password several weeks ago, it is possible the local synchronization data became corrupted or incomplete.
Before doing so, please confirm whether this is the only account configured in Outlook and whether any data is stored only locally in Outlook and not on the server.