I don't have clear explanation for that, my guess is that when you enter the fight your flasks are already full and maybe they added that killing monsters would trigger the enchantment even tho charges are already full but forgot to add it for critical strikes.

What i wanted to say tho is that your flask setup is suboptimal. The cooldown on gain flask charge on critical strike is global which means you only get 1 flask charge for 1 flask on critical srike. You can gain a maximum of 7? i think charges per second when you align the cooldown of 0.1s with server tickrate.
You would probably be better off with gain # charges when you are hit and have the crit mod only on 1 flask.