I’m going to jump in and say your “deals the same damage” isn’t a fair comparison.
It looks like your characters and the opponent’s all have wildly different health/protection pools, so I’m extrapolating there’s a major difference in gear levels and perhaps modding too. Look at the damage of your Sortie compared to your IG, there’s almost a 500% difference there.
Your lower-geared characters will take more damage than higher ones due to defense/armor levels, on top of possible unique abilities affecting damage mechanics.
There aren’t any other posts in the forum calling this out as a problem, so I’m guessing the mechanic is working as intended.