Mail InBox

OK, I just solved it myself. There is a "Filtered by (unread)" link at the top of the Sender Panel to the right of the "Sort by (date)" that must have been set accidentally. Once I used the drop-down menu to uncheck "Unread", the InBox magically revealed all of its treasures.

I hope you will add this fix to your bag of tricks as the go-to for this issue before suggesting reboots in the future. Thank you for your assistance.

Tried this suggestion twice. Doesn't change (see screenshot). There is mail in BOTH mailboxes under the InBox.

I noticed an "Issue Resolved" under "iCloud Mail" in the System Status window ... was this non-visibility of all inBox mail the problem? ... Then it is NOT resolved!

Each one shows the mail items within their respective mailbox.

Viewing change only alters the appearance of the Sender and Content panels, NOT the Mailbox appearance.