E.L.F brushes are notorious for this…just saying. If you get water under the ferrule (metal part holding the bristles to the handle) it will start to break down the glue and the handle will separate from the bristles and ferrule.
An easy way to fix this — lash glue!
Just put some inside the ferrule like so
and stick it back onto the handle.
You can also use superglue if you have it. I just have the ability to never have superglue when I need it. -_-‘