1 of 3 external USB drive volumes missing in Finder sidebar?

I backup 2 iMacs and 1 Windows PC on one external USB hard drive on even months and to another on odd months. There are 3 partitions containing one volume on each drive. I only connect a drive to a computer when it’s time to do a backup. Until this month, Finder showed all 3 volumes in the sidebar for either drive. This month, it only shows 2 for either drive on either iMac. But if I select the iMac in the Sidebar’s Locations, Finder displays all 3 volumes in the iMac’s contents. If I click the volume missing in the Sidebar, Finder displays the contents of that volume with no complaint. Files on that volume Open normally. Any idea why the volume doesn’t appear in the Sidebar?

macOS version on both iMacs is 12.6. I don't know what it was a month ago which is the last time I know Finder displayed all 3 volumes.

The structure of the two USB hard drives is somewhat different:

In Sidebar?

Drive1 Partition 1 macOS Extended (Journaled) Yes

Partition 2 APFS No

Partition 3 Windows NTFS Yes

Drive2 Partition 1 APFS No

Partition 2 APFS Yes

Partition 3 Windows NTFS Yes