IIRC, the original problem was that there were 2 probes, and that one of them showed a slowing of speed. Then, later, the second one showed some slowing.
There are about 20 billion ‘things’ in space around these probes that could affect their speed.
One micro-asteroid hitting the probe could cause a change in speed. A change in the density of background hydrogen could cause it. Gravitational influences from objects we can’t even see, might cause it.
Saying the Sun has some ‘other’ kind of ‘force’ that is pulling these probes ‘backward’, seems almost ludicrous.
More likely is that it is something else, and we don’t have the equipment on board the probe that would sense ‘what’ and ‘where’ the something else is.
See #15.