Hello Pavel,
This may be because you have added a new property to the GW Model but missed adding the same to the structure which is bound.
At the runtime internally there would be a check which maps properties to the structure which is bound to it.
In the MPC class you will see a structure which is bound as below.
lo_entity_type->bind_structure( '<Your Structure Name>').
You need to add the field in this structure as well.
Check this structure and add that particular field.
Regards,
Ashwin