Fullback Blocking Versus QB Spy

I have repeatedly run into a bug in how fullbacks interact out of the Pro I set when blocking Duos to the left-hand side. Typically, the blocker should block the defender to the outside, setting a running lane to where the LT just left. Against QB Spy plays, the fullback starts running right instead of left and just ends up virtually tackling the RB.  Is there any way to avoid this bug? I have repeated this bug 20+ times with zero variation. Left side duos is a major part of my rushing game, and I would hate to have this bug pop up at a vital moment.