I think this is a bug. First the workaround(s):
Next, based on a quick review, it looks like the input boxes are getting attributes called "focusindex" instead of "tabindex", which I believe is incorrect. The buttons get "tabindex". I can't find any references to "focusindex" being an attribute that is respected by browsers. I will submit a Mendix support ticket about this issue.
EDIT: Also, that is a very strange use of tables. I would recommend you use the Mendix layout grid for this purpose, or at the very least you shouldn't need 2 tables inside of each other to accomplish your design.
EDIT 2: Support request 49300