Hi,
you have a custom field(on the opportunity entity I suppose), which is displayed both on form and on the BPF.
So i think that your problem here is that when an opportunity is closed (as won or lost i don't mind), your custom status field is not "synchronized" (as the opportunity closed cannot be edited).
So my solution here is, forget about the error message, just use a workflow to create a process that triggers on the system status fields change and then updated and "synchronized" your custom status.
something like this:

You can then add a condition like
If status equals won then
Update Opportunity, set custom status to won ecc.
Hope it helps
Marco