I've been hot for the protobelt since its release, and I've been using it quite a bit. On three separate occasions, however, I've noticed that the stun was not working if I cast Tibbers during a protobelt-dash. I've tested this three times. I'm sure I hit, as the damage happened, and I'm sure I had stun, as the counter reset after tibbers landed.
Is this intentional? I could not find any text or tooltip information that gave implications that this was not supposed to work, and the patch notes even mentioned Annie specifically when talking about the belt.