I'm sure there's less lift in a slip, because it's going down like a lift.
Except that if you weigh yourself in a lift descending at a steady rate, the answer will come out the same as on solid ground. It's only when the lift starts going down that you will temporarily weigh less, or when you slow down your descent at the bottom where you will transiently weigh heavier.
I wonder whether this could be a real reason to expect stalling speed to increase when converting from a slip to level flight - your rate of descent is liable to decrease in level flight, so whilst transitioning you will require more lift and therefore a greater angle of attack than when in an established glide. You're effectively pulling out of a dive.