To see the actual damage roll, do not look at the weapon, but your attack option in the hotbar/equivalent. In most cases, it will display all buffs applying to that action. It's a bit more tricky with spells such as magic missile, but crossbow damage rolls should be accurately reflected. This is largely helpful for comparing weapons and keeping track of which benefits come from where. For example, there's a ring that adds a flat +2 acid damage to rolls as early as Act 1 -- it wouldn't be good if players couldn't tell if that trait was innate to a weapon, or the ring. Like the + 2 acid damage, the +10 flat damage from the feat isn't innate to the weapon and is hence not displayed in inventory. It's a design choice.