that makes sense as even though pb1 is not connected, it is still "listening" as it is active in the patch object.
I have taken to stripping unused ports from my objects for this reason.
From the Init code:
pwmStart(&PWMD3, &pwmcfg);
palSetPadMode(GPIOA, 6, PAL_MODE_ALTERNATE(2));
palSetPadMode(GPIOA, 7, PAL_MODE_ALTERNATE(2));
palSetPadMode(GPIOB, 0, PAL_MODE_ALTERNATE(2));
palSetPadMode(GPIOB, 1, PAL_MODE_ALTERNATE(2));