Here is the reply I just got to the ticket I submitted to Canvas Support:

"We made a change recently where if you have an invalid access token, no apps would show rather than all apps. So you probably have an invalid token. You will want to either remove your token from that "Manage App List" button, or set a new one https://community.canvaslms.com/t5/Admin-Guide/How-do-I-manage-an-Edu-App-Center-allow-list-in-Canva...

Because I didn't have any previous information to work off of from whoever at my institution initially set up our external apps, it looks like I now have to re-set up all the apps in the EduAppCenter. I generated a new token, and will now have to set up "Organizations" and the apps. Here are some more articles I was prompted to use:

https://community.canvaslms.com/t5/Admin-Guide/How-do-I-manage-an-allow-list-in-the-Edu-App-Center/t...

https://community.canvaslms.com/t5/Admin-Guide/How-do-I-create-and-manage-organizations-in-the-Edu-A...

Post-script:  I followed the steps as outlined in these admin guide articles and it worked. We regained our access to all the apps, both on our sub-account level and the individual course level.