Hmm, you're right, we've fixed this in 4.0.05-beta. There when auto key is off, undo/redo works as in 3.8.99. When auto key is on though, undo/redo applies the animation so the skeleton matches what you see in the dopesheet and graph. In 3.8.99 it didn't apply the animation until you scrubbed the timeline, which made seeing what was going on a little odd.
We'll release 4.0.05-beta in a few minutes. Cheers! :beer:
