Whenever I use an AOE and hit a critical the critical only applies to one of the enemies and it hits base damage on the rest. I am currently working on the Wysteria side world so I know these enemies can't block it. I've seen some people mention this as well, and just wanted to put it out there to hope for a fix soon.
This is Intended. when the game checks if you critical, it doesn't check once for AOE's. Every AOE checks up to 4 different times, (keyword "different") For attacking enemies. you can critical 2 out of 4 enemies, 1 out of 4, 3 out of 4, and nothing is wrong with your critical. of course you can critical all the monsters if you are lucky. This is caused by the confusion with critical system, because it was changed in a past update.
You don't just critical every monster when the critical text comes up. Right now there's no way to see which monsters you critical on unless you see different damage numbers. (thus causing this confusion)
However, please play with a peace of mind that you are not losing damage on your AOE's. If you don't do critical damage to a monster, It's because you failed to roll a critical on said monster.
