Regression: add_field is no longer enriching events

[donoghuc]

When upgrading from 3.2.2 to 3.3.0 pipelines which use an add_field silently fail to enrich the envent.

A PR was filed detailing where this regression may have stemed from. This issue tracks restoring the expected behavior and ensuring event enrichment works as expected. If possible also extend test coverage to validate this use case and prevent further regressions.