Answer
This points to the workflow's own logic, not the platform, once a validation script is extended — either the workflow's gateway is now resolving to its success path when it shouldn't (check the new custom field reference for something like a case-sensitivity or field-name mismatch), or an error inside the workflow is occurring before it reaches the platform's failure-end-event, which would also result in the UI never seeing a proper validation failure; re-verify the new field reference in the script first, since that's the most likely point the behavior changed.