I'm running ACM 2 code from todays trunk ( r2336 ).
I'm doing PID tuning in APM Planner 0.4.30 in the "PID Config - AC2 MAVLINK" window.
when I try to save some of my modified PID settings, it behaves like it's worked ( I get the "DONE" ), but then re-opening the PID window and re-reading the settings gives me the OLD PID setting/s.
Some settings work OK though!
changing Stabalize Roll Pid -> P does NOT save.
however ( eg):
changing Acro Roll Pid -> P saves fine.
changing Other Mixes->Stabilize Damp saves fine.
It seems that nearly all the settings I've tried will save *except* for the "Stabalize Roll Pid" ( which by the way is spelt wrong in the GCS, so I've spelt it the same here).
I cant replicate your problem Buzz. I fixed the spelling also