Sudden drop of Lift force during transient simulation
 

I'm simulating a small wing (40mm chord), with Transition STT at hight angles of attack, and I'm getting some strange drop in lift (and rise in drag).

Time step is 0.005s.
Y+ is <1 everywhere.
Also, I'm new with low Re simulations, should I consider viscosity as a constant or should I use power law or sutherland?