The Cl versus alpha curve of an aerofoil is dependent upon many factors. One of these is camber. When any flap is extended, the camber increases; this modifies the lift curve slope; effectively moving the Cl max point 'up and to the left' on the overall Cl versus alpha graph. The aerofoil will then stall at a lower value of alpha with flap extended than when clean, but at a higher Cl max value. As Cl max is higher with flap extended, at the same lift value (L=W) at the stall, Cl 1/2 rho v**2 x S, it follows that v will be lower than for a clean aerofoil.
This should be explained to PPL students prior to Stalling 2..........