It's likely calling a different BAdI to that which you are calling...Have you implemented the right BAdI? The system method BADI is found through path:
MDGIMG -> Master Data Governance -> General Settings -> Process Modelling -> Workflow -> Rule-Based-Workflow -> Business Add Ins -> BAdI: Calling of System Method for Rule-based-Workflow
If not - I'd suggest finding the code that calls the BAdI to find out exactly which one it is calling.