This is libGdx-part. According to X/Y-values revert as it was before. Then just delete all "actor.getWidth() / 2f" & "actor.getHeight() / 2f" additions and subtractions in begin() and update().
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That would have been my answer if there (and in many other Actions) wasn't the Formula object which calculates in UserDimension units. We should discuss this personally according to a better separation of libGdx.