This guy bug reported it some time ago and it was proved by QAJonny that it actually isn't a bug in post #12 explaining exactly how the PAS works on Planets:
"Hey Everyone, Malic a7 MickyG1982 have hit the nail on the head with the answers. I have spoken to the design team and the programmer behind the feature and they say "the fiction is that in near-planet flight the cooling and power for the thrusters all gets re-arranged to make the ventral ones strong enough to fight gravity, this leaves the lateral and dorsal thrusters seriously impaired."