Answer
Carefully re-check for an exact name mismatch between the job's Method Attribute field name and its Source Mapping field name — for example, a field name that's suffixed differently, such as one side having a _DB suffix that the other side is missing, between the two configuration points. Even a single small naming mismatch between these two fields will silently cause that one specific field to be skipped during the actual update, with no visible error anywhere in the migration job's results, while every correctly-matched field continues to update normally on the same records.