Sync rule - Allow Null to destination is allowing null even if unchecked

I added a new rules extension code to AD MA and after that I ran AD FS. AD FS triggered allow null values to be passed to various attributes in AD even if Allow null is not selected in sync rule flow. What might have caused this issue? This is the code I added.

        void IMASynchronization.MapAttributesForImport( string FlowRuleName, CSEntry csentry, MVEntry mventry)
        {
            switch (FlowRuleName)
            {
                case "homeMDB":
                    {
                        if (csentry["homeMDB"].IsPresent)
                        {
                            if (mventry["employeeID"].IsPresent)
                            {
                                mventry["homeMDB"].Value = csentry["homeMDB"].Value.ToString();
                            }
                        }
                    }
                    break;
            }

        }