When you use the boosted dirt to break a dirt or grass block, you get +1 dirt and +1 grass per level of boost (so +3 of each at B3). The double drops multiplier only applies to the extra blocks of the type you broke. So if I break a grass Block while holding a B3 Dirt, it gives me 8 dirt and 3 grass blocks, when it should be giving me 8 dirt and 6 grass blocks. Tested this multiple times immediately before I posted this message.
We just tested this and it looks like everything is working correctly. I think what you have experienced is a mismatch in dd drop chances. When your double-drop rolls for dirt, it does not also roll for grass block at the same time. If your dd chance for grass block is lower than dirt (and it will be if all you have is a boosted dirt) then grass block will not double at the same time as dirt.
