When I shapeshift into cat form and go stealth, I don't get a kitty stealth action bar anymore. Is this a bug in nUI?
No... it is not a bug... it's how the Blizzard action bar engine works.
Odds are that prior to using nUI you were using another bar mod like Bongos or Bartender that offered you the ability to create special bars. However, the Bliz engine does not have a kitty stealth bar. You can verify that by disabling all mods and logging onto your druid, shifting to kitty form and then stealthing... no special bar.
In due time, nUI will extend its action bar capabilities to include creating special bars like this. But for now, in order to ensure maximum compatibility, nUI adheres strictly to the rules Bliz has defined for the primary action bar's behavior.
What people don't get is that I am, ultimately, an artist at heart.
My brush has two colors, 1 and 0, and my canvas is made of silicon.