If you have a planned completion date field in your custom form, it sounds like someone made a custom field and called it Planned Completion Date. So basically you have two fields with the same name.
In a recent user group meeting on system maintenance, we recommend against creating custom fields that mirror Workfront default fields -- you can read about it more, here:
It really depends on why you have that field there. If it was put in for a reason, you could at least rename the field to get closer to the meaning of why they are there and remove the ambiguity.
Some examples of names we've opted to use instead are "Marketing Planned Completion Date", or "Requested Completion Date" or something along those lines. We often do this because we want a record of what the requestor originally asked for, but the Planned Completion Date is ours to fill in and change as needed. i.e. "we need a historical record, so we made a second date field."
But if your custom form was built thinking to put Workfront's Planned Completion Date in a more convenient spot, then the thinking was incorrect (this is not how Workfront works), and you would be better off if you deleted the field and just coached the users on where the find the planned completion date.
If having the field where you want it is important, you may want to use a calculated field that reflects the planned completion date so you can place it where you want it.