Hi,
it seems strange to me, non-modifiable fields should behave like modifiable fields when writing into an entity.
A problem may be introduced if the non-modifiable field is not entity based when instead the rest of the Form is. In this case you have to manually bind the field with the entity property; otherwise you can delete the field and use the 'Entity Field Wizard' to add the field and then from its properties you can mark it as non-modifiable.
If the field is already entity based I suggest you to run the 'Find Model Problems' procedure and/or debug the web application.
Regards,
Lucio