No idea what the instructions say. My guess is that if the whole thing is in warm water then the water will get all the way to the glue which is below the top of the handle. Over time that may cause swelling, as Phrank was saying, and if the hole in the handle was also a little bit of a snug fit those two things might cause the handle to crack. I guess if the brush is a daily user that may also not allow it to dry completely between uses and so adding to the risk of swelling the knot.
I have seen brushes with vertical cracks from what I had assumed was the knot swelling. I have not seen one with cracks radiating out from the base of the crack. That is the only thing I can thick of that may cause cracking in a shave brush handle besides dropping it. Sorry to see that happen to a nice brush.
Bob