Cosmic Brainz cannot carry a Charged Punch and jump, which you've probably noticed, but the offshoot of that you can lose the charge if your character becomes airborne, like when dropping from one terrain level down to another or when you are stunned or knocked back.
I'm just a Player like you so I can't explain why the game might drop the charged punch one time and not the other. Pretty much every other character, mostly Plants, with a charged attack loses the ability to jump when building or holding a charge and they typically get some sort of small movement penalty.
If anything, based on the ehavior of the other characters with charged attacks, I think if there is a glitch or technical issue at work it's probably allowing Cozmic Brainz to carry a charge when he shouldn't be as opposed to causing him to lose it when he shouldn't but we could probably also maybe blame server communication issues as well for letting the charge carry considering that we encounter that sort of thing all the time.
All I can say for certain is that I have no trouble holding a charged punch when Cozmic Brainz has both feet firmly on the ground but I have noted that the charge tends to drop when he become airborne or his feet otherwise leave the ground for more than a moment.
