Hi, I realize this is an old thread, but I'm having same issue and have not found solution. I do have an additional clue, though!
Tab key input in text box inserts old-fashioned tab space in the box, ONLY after I have lingered in a text box for a minute or two, so there may be some sort of time-out issue occurring. If I swiftly tab through the fields while entering data in them, I never have a problem.
My settings for ALL the text-box controls all seem to be correct:
TabKeyBehavior is set to False; and the related control, MultiLine, is also set to False.
still a mystery ..