Battle Master combat maneuver damage dice is not rolled twice when you score a critical strike, in the game's current state (patch5).
If it is a bug then it should be fixed. If it is not a bug then it needs to be redesigned so that the 1d8 damage die should be rolled twice when the attack is critical.
" If the attack involves other damage dice, such as from the rogue's Sneak Attack feature, you roll those dice twice as well" PHB pg. 196