I just saw this, so thought I'd quote it here for the benefit of anyone who ends up with a similar problem (the Arcana head coming off when removing it from Brainstorm - popping off the ball of the neck because the fit is too tight in the Brainstorm cavity)

For those of you with broken headmasters, there is a very easy fix I found. If the head separates, use something like a pin, sliding it into the small empty spot on the back of the head to lever it out. Once that is done, super glue the head back on to the body (but only put the glue on the headmaster's head, not the body, so he can still use his ball joint) then very carefully shave down the left and right sides of his head slightly. If you shave down too far, don't worry, just apply clear nail polish to the head, and he'll be fine! It worked perfectly for mine, and I'm certain it will for others!
Not sure if anything actually breaks, or if it is just the head popping off the ball joint, but maybe a preventative measure could be done first, relating to trimming the sides of the head and applying a coat of nail-polish onto the ball joint to make its friction more than the friction of the head into the Brainstorm cavity.

I don't have this yet, so am talking blind here... so when I eventually do, or if someone else gets it first, something more accurate can be added if required.