Since it was a specific akonadi issue, I opened the issue in the KDE support forums. It took a while, but tonight I finally tracked it down with help from wolfi323. For some reason, the system didn't update some of the packages associated with akonadi. Specifically, I found this list of packages stuck at ver. 16.08.3 while everything else was either from ver 4 (not relevant) or was on ver. 16.12.3.
- akonadi
akonadi-kde
akonadi-mime
akonadi-search
akonadiconsole
lib64kf5akonadisearchcore5
lib64kf5akonadisearchxapian5
I used drakrpm to uninstall all of those packages and then installed the 16.12.3 versions. After a reboot everything worked as expected.
Not sure what caused that mismatch, but it's fixed now and I've got very few things not working to my satisfaction. But that's for another message.
Just wanted to make that available for the record.