Hi Vamshi,
What do you mean by metadata field? Why are you using an invisible field to capture this information? So that it can update the PD directly without further mapping code?
As far as I know if the field is present in the screen, even if it's invisible and the data is available in the context, update will have the field data, but will get saved to the DB only if you have the same field available in the corresponding bbp_pd table.
Regards,
Vidya