I see this sort of thing done on quite a number of MUGEN characters and I figured I'd make a thing about it. I posted this on twitter, but figured I'd post it around also.
When making a characters get-hit animations, the hurt-boxes should stay consistent with the base pose from which the get-hit animation is based on. (Standing, crouching, in-air)
When you make you hitboxes pull away in attempt to be accurate to the animation, this can cause combos to drop that would otherwise be consistent from character to character. The same also applies to a character guarding animations.
Even in the air and in falling states, these hurtboxes should stay a consistent shape, centered on the character's center of mass. They should not change to cover the character from frame to frame.
Something unfortunate is that KungFu Man, the default example character for MUGEN, gets this concept wrong as well. Not so much on the ground, but in the air for sure and I think this kind of contributes to this misunderstanding of how to hurt-box.
There are some games that handle hut-boxes not so well. Like Smash Bros. As much as I love that game, I dislike the way it handles the hurt-boxes, binding them to the character modles, rather than making them an independent entity, which creates a lot of match-up specific shenanigans at times. Even Street Fighter 3 also did frame-by-frame hurt-boxes also and it causes some weird interactions in certain match-ups.
It's generally a good idea to not be so accurate to the character's movements from frame to frame. You don't need to cover every pixel nor do you need to worry about boxes running off of the sprites a bit. You generally want the SIMPLEST and FEWEST POSSIBLE boxes to represent your character's body.
This is all ideal for keeping balancing easy and match-ups more consistent.