A Data Mapper returns different results for an integration user than for you. What do you investigate?

Data Mappers execute in the calling user's context, so differences between users are a permissions question until proven otherwise.

1. Object permissions — does the profile or permission set grant read (or create/edit for a Load) on every object in the definition, including related ones?
2. Field-level security — fields the user cannot see are omitted from the output rather than raising an error, which is exactly the silent-partial-result symptom being described.
3. Record-level sharing — the user may have object access but not visibility of the specific records, so an Extract returns an empty list where yours returned rows.
4. Row filters and inputs — confirm the input value being passed is genuinely the same. An integration user often arrives through a different entry point with different context.
5. Reproduce it properly: log in as the affected user or use a permission-set-matched test user. Diagnosing this from a System Administrator session is how the problem stayed open for a week.