I am afraid it does not work even if for simple text input control: I inserted 4 text input controls... changed their positions, assigned tabindex from 1 to 4 in order I needed... unpredictable navigation route when pressing Tab (or Shift+Tab).
It gets more complicated in Edit Form, where we have data cards (without tabindex property)...
This is a very basic function (existing in Access 20 years ago), and very important for streamlining data entry, at least for the generation that prefers keyboard shortcuts to touching a screen... 
I hope I missed something, so this essential functionality is there but I could not benefit from it...