What I think is going on here with MSFS is that the changing angle of attack of the wing along the aileron is not sufficiently taken into account. As you deflect an aileron down you change the chord line which affects your angel of attack since the AoA is measured as the angle between the relative wind and the chord line. In the case where you are in a slip with full right rudder you are rolling left with the yoke, this causes the left wing to raise its aileron, this decreases the AoA, and on the right wing you drop the right aileron to lift that wing and in doing so increase the AoA. So because of the difference in AoA between the wings, plus any wind blanking that may happen as a result of the fuselage blocking some wind from the right wing the left wing should stall first. Hopefully that made sense.
I have taught aerodynamics for ATPL students but you are exactly right, I didn’t want to write it down in such detail as I assume the devs. have some knowledge about aerodynamics themselves.