Date field format not sticking for some sumbissions

[roattw]

Have a date field with the Pattern field set as date{DD-MMM-YYYY} (for Display, Edit, Validation and Data):

This form is published via Acrobat.com and Tracker.  When loading repsonses in Tracker it notifies me that some of the field formats dont match (indicated in tracker as 'Has Different Fields'), which I think are these form date fields.  Its set to allow the user to enter their own values (with the suggested format indicated) but if people dont enter it correctly its not getting converted correctly somehow.  How do I troubleshoot this, everything appears set correctly.

Thanks!