Currently, it seems that the Go driver is quite strict in enforcing RFC3339 format for its dates, but the docs mention that dates conform to the more looser superset of ISO8601.

The Python driver seems to be more lenient with this and allows valid ISO8601, but is strict in its output and will give back RFC3339.

This is currently affecting a customer for Atlas Data Lake, who is using ISO8601 date format that is in conflict with RFC3339, and is causing errors when using the Go driver to parse their JSON.