It could be that the smaller 650b is resulting is less trail accentuating the flop. I'm not fully clued up on how one affects the other though. I have a touch too much flop on my conversion, it self steers a bit too much on sharp corners, but that's because the fork is a bit too long for the frame and the headangle is already slightly slack. It equates to a decent ride as long as you're used to it and don't suddenly slam it in to corners.
I guess trail (more=more straight line stability) balances flop (more= more power steering effect) and we're both a bit too far in to the flop direction. This can be a desirable effect but it sounds like it's a bit too much on your bike for the style of riding you're doing, as I say I'm no expert and I'm not even sure how pneumatic trail plays in to this (or whether it is even real!).