Content of custom fields on Users is not migrating from D7 to D8 on the first pass. It does migrate if Users (and nothing else) is rolled back and re-migrated. This appears to be because the fields and field instances are not migrated until after the users.
Ensure fields and field instances are migrated before users.
Add d7_field_instance (and consequently d7_field) as a migration dependency of d7_user (as it is for d7_node).