Hi Eric,
Looks like that form is created off a SharePoint list, right? The schema is locked. I tried to change the tab order myself in the form and it doesn't work as I would expect. One of the fields was -1 for tab order. I tried to order them all as they should and it wouldn't work.
Did you move the order around from the order that they are by default in the SharePoint list? I wonder if that has anything to do with it? Seems like something about this be bound to a SharePoint list might be causing the issue.
Ted