In G4LowEnergyPolarizedCompton::GetPerpendicularPolarization
We see the formula:
// p = a + (a o n)/(n o n)*n
return gammaPolarization +
gammaPolarization.dot(gammaDirection)/gammaDirection.dot(gammaDirection)*gammaDirection;

I think the formula should read:
// p = a - (a o n)/(n o n)*n

so that
p o n = a o n - a o n / (n o n) * (n o n)= 0
What the function try to achive

with the formula above
p o n !=0

Correct code should be:
return gammaPolarization -
gammaPolarization.dot(gammaDirection)/gammaDirection.dot(gammaDirection)*gammaDirection;