Hi Andrea,
1. You should have the DDLB_CLICK event handler in the IMPL class in order to trigger the round trip when you select the drop down values in LOB field. It should be an empty event handler method. Not necessary to have any coding.
2. By any chance, if you have "if rv_valuehelp_descriptor is bound" in field 2 GET_V method, remove/ handle it properly.
Reason: System may skip the ddlb fill logic, if the rv_valuehelp_descriptor instance is bound. In your case, the get_v should gets executed completely every time.
3. it is good to have, the following piece of code in field 2 portfolio element get_p method
CASE iv_property.
WHEN if_bsp_wd_model_setter_getter~fp_fieldtype.
rv_value = cl_bsp_dlc_view_descriptor=>field_type_picklist.
ENDCASE.
If it is a server event trigger issue, ideally the point 1 will fix the issue.
Try it and let us know the progress.
Regards,
Bharathy