I had a close issue of the same magnitude.
Tab order worked in Preview PDF
Cause: I discovered that a HyperLink e-mail address that was put in while designing the form in MS WORD was the cause
Solution: Changed the hyperlink to regular text. I made it blue and underlined it (Looks like a hyperlink). Saved form as PDF
and opened it up back in Adobe Live Cycle. Customized tab order slightly by going to Window > Tab Order and making custom modifications.
Issue is resolved.