I’m Andi from E2E Support.
I had a quick look into your model. It appears to me, that you have created the RFC interface manually and not applied the recommended way to implement SAP RFC interfaces with the RFC WSDL generator.
I recommend that you use this utility. You can download it here and you find also documentation how to use it.
I was also able to reproduce the issue you described and I will forward this to our developers for further investigations. But as said, please try with our RFC WSDL utility. It lets you implement RFC interfaces very fast and reliable with E2E.