This was resolved...but why it was resolved remains a mystery.  We simply re-provisioned by turning off and then on the SCIM provisioning in the Azure portal for the Databricks SCIM connector (which is supposed to cause a full re-provisioning, but didn't seem to do that last week).

There are some good pieces of information here: Configure SCIM provisioning using Microsoft Entra ID (Azure Active Directory) - Azure Databricks | M...

...in particular, it appears from the tips in that link that you can manually add back users inside of Databricks that were mistakenly deleted inside Databricks, and the synching from the SCIM connector should still work fine.

Finally, there is a check-box in the provisioning section in Azure portal for the Databricks SCIM connector, which say "prevent accidental deletion".  We didn't check that box, but wondered what it does.