This is something that I have never gotten to the bottom of. Hopefully, somebody on this thread will come up with the answer.
Kaypam. I agree that the "Angle" of the stall changes depending on what surface moves i.e. Slats have a different effect than Flaps.
However, flap or slat extension should always result in the stall speed becoming lower. Therefore Vls (speed) should always reduce. Hence why I don't understand why Vls temporarily increases.