I gathered from what jmillikin said that it is certainly a bug, but it was probably not reported during the testing period for the same reason jmillikin didn't report it: it appears to be hard to reproduce.

Is there a tool (akin to xev) that would display which process currently has control of the mouse? I would think that tracking down the process that has grabbed the mouse would be the necessary first step toward stopping it from doing so.