It is a bug. I had the same thing with just a bow. Fighting the Heavy Orc near where you find you "familiar sword", I use the run-and-gun tactic to kill the orc. Whenever my Stamina bar was depleated, if I clicked to run somewhere at just the right moment, the Stamina bar would refill. I think there's a small mathmatical error in the code and when an illegal amount of Stamina is preent, it resets to full. Hopefully RiftRunner will fix this.
And in regards to testing, they didn't have much time to test because CDV pushed the relesae. They have a large in-house beta test for RiftRunner occuring soon, far better than any of Divine Divinity's testing.
