/api/dashboards/uid/ doesn't bring datasource id

When I use /api/dashboards/uid/xyz123 API, I get all dashboard definitions, except the datasources’ ids. It returns dashboard json with “datasource”: null,
If I export it via GUI (share->export) it does have datasources’ ids.
Pls advise.

Looks like I found an answer by myself. Whenever the default datasource is used, it appears as null in dashboard json.