I found this bug by complete accident.
I dropped a whole stack of block on a light pressure plate and it only gave me signal with strength 1 and thought it's not supposed to work like that. Then I dropped 3 separate blocks on it and got signal with strength 3 but after a few second blocks combined into one item and signal strength decreased to 1 again. I haven't tested but I suppose it also affects Heavy pressure plates as well.
Take a look at screenshots for better understanding