smart mailbox folder does not appear after creation

I tried and at first I could not see the created smart mailbox folder.

Then I did something: I selected a few smart mailboxes in the sidebar, and went on to create a smart mailbox folder, and it showed up. Now it seems new smart mailbox folders are appearing normally. You may want to try this trick.

Hello johngar,

Thank you for using Apple Support Communities! I understand that a Smart Mailbox that you have created is not appearing in the Mail sidebar on your Mac. You've come to the right place for help.

To help further isolate potential causes of this behavior, I would like to start by having you create another Smart Mailbox using different criteria:

Use Smart Mailboxes in Mail on Mac - Apple Support

Does this new Smart Mailbox appear? Do you have any other existing Smart Mailboxes that appear?

Cheers!