Trouble getting one field to pull through from an external (legacy) form


We've built an external form connector, consisting of a "legacy form" that captures data entered by the user on a form on our website.  All the fields are working fine EXCEPT for ONE free form text field!  

All our other text fields are working properly, and this one is configured the same way.  The only difference is that this field - which is not working - is a custom field (ex. not default).

Instead of pulling in the user's response, all that we see is in in terms of data for this field is the word "true", both in the "event" and the "lead/contact details" view...

Has anyone seen this before?