Bosch cuts power when combined with shimano di2 shifters.
I did read somewhere that even without Di2, the Bosch Gen4 system 'notices' the slight torque difference when changing gears and reduces output. I can't say I have noticed this as it is habit that I ease off when changing and I really struggle not to do it in order to test this theory. I have a feeling that even if true, it would work better with upshifts than down as the torque drop is more noticeable to the motor when upshifting.
As someone mentioned above, I would really like to see a total and deliberate shutoff when changing (like a motorbike/car) and it seems Di2 is the answer (I presume it sends a signal to the motor?) but out of my price range at the mo. Until then, the best we can do is still try and ease off a bit (which is hard on a hill I grant you) or try and speed up just before easing off for the gear change but again this isn't easy if the hill is ruddy steep. I am OCD about stressing my drivetrain but sometimes it just has to happen