Actually i believe the problem is that impulse only regognizes the stable 1.09e edition (and previous stables of course) as the versions that can get the patch for every new beta update.
But it seems that if you have already an other beta, impulse doesnt regognize it and tries to redownload the whole game, since it seems that it doesnt understand or it doesnt work for the atch to be applied to an other beta version to the new one.
So one of two reasons. Either each new beta patch is compatible only with stable editions or its too much trouble to do a beta patch for each of the other beta edition, thus easier for us just to revert back to stable and reupdate.
Or it is impulse either not regognizing each of the previous betas that should be applied the patch properly or just someone forgets that they should inform the impulse patcher to inlude also every beta version to be update along with the stable ones.