Or more precisely, we don't know whether it has or not, because the data are not precise enough for Voyager.

The Pioneer craft are spin-stabilized only at this point, no thruster firings at all.

The three-axis stablized Voyagers are still firing thrusters for attitude correction now and then, and for some reason that means they can't get precise enough velocity measurements. I'm guessing the uncertainty in the thrust imparted by each thruster pulse is such that the accumulated uncertainly in the expected velocity is larger than the magnitude of the effect on Pioneer. That is, even if the doppler tracking of their actual velocity is accurate enough, we don't know precisely enough what their velocity *should* be to say that their actual velocity is different from the expected value. Whatever is going on with Pioneer, it is a very small effect.