Changes register item processing on connector failed with reason Cannot process a DateTime of type 'Unspecified'

The error Changes register item processing on connector TechOne Person failed with reason Cannot process a DateTime of type 'Unspecified' is logged for an adapter which has a join to another connector with a Timestamp field that was populated by PowerShell when that adapter's base connector imports a new or updated entity, even when that Timestamp field has DateTime kind 'Utc' or 'Local'.

My config has two PowerShell connectors, TechOne Person and TechOne Position.  The Position connector has two fields StartTimestampUTC and EndTimestampUTC which are set to valid DateTime values, of kind 'Utc'.  There is one adapter, TechOne Person, for which the Person connector is a base, and the Position connector is a Join transform with StartTimestampUTC and EndTimestampUTC fields both mapped into adapter fields.

When a new record is present in the Person connector import, reflection causes the above error message to be logged, but the adapter record is created correctly nevertheless.  When a subsequent Generate Changes is run on the adapter, the error is not logged.  If the record is reimported with updated values, the error is logged.