Designing forms with BCM is not officially supported. The problem
you've run into is one reason.
Some of the fields in Outlook forms are computed on the fly, their data
is not stored, and they require special handling which the Outlook
forms designer doesn't support very well.
Not to be negative but this kind of thing exists because BCM was likely
cobbled together quickly without tremendous attention to such details in the
design. There are numerous of these types of oversights that are just not
easily explainable.