I came across this method whilst I was drying some brushes. I had just finished my sister in laws make up and needed to then do mine, I quickly washed the brushes and left them to dry on a hot plate of my hair crimper.
It got very hot and loosened the glue. After an initial, uhoh, I left it to cool and it reset with no problems. It didn't take a genius to work out that I could take a separated brush head with glue still inside and handle and use heat to mend them together again! You are very welcome to call me a genius but I am just glad that I can save brushes from landfill and give them a new lease of life.
A note about plastic brushes, these are not suitable for this type of mend, I advise you to use glue.