I just tested this in Explorer Mode, in The Underkeep dungeon, as a Dragoon with a 745 weapon (from Zalenia Ex):

The striking dummy takes 1158534 damage.

That's 1,158,534 damage. So it seems like you are mistaken, unless the bug is specific, in a specific duty, in specific circumstances that SE did not consider, or unless the damage stated in the battle log is incorrect somehow.

In order to test if the damage received differs from the damage dealt (such as due to enemy mitigation), I switched it from "damage dealt" to "damage received by engaged enemies":

The result is 1,153,762 damage.