I have noticed that the Action Bar at the bottom of the screen was missing, at the start of an interactive battle, on a number of occasions. Curiously, I have never had it disappear, once a battle had begun, (if that is what you are describing.) As Glazunov1 notes, I believe this is a recognized bug; and presumably it will be fixed in the next beta. (And of course, you were right, to try to report it.)
But, as an aside, are you aware that it IS usually possible to recover the Action Bar ? If your hit the lower right button, that plays the combat out, automatically, and then hit it a second time, after your sovereign or one champion has moved, the combat bar will usually re-appear. The bad news is that you have lost control for a moment; but the good news that you can regain control, and then fight the rest of the battle as you want to do it .
At least, that is better than watching the computer (usually) lose the battle for you! Obviously this needs to be fixed; but on the plus side, the damage to your play/fun can usually be limited ...
