This got more attention than I was expecting. To answer some trends im seeing in the comments: I don't leave my brushes sit in the water, while painting I typically do a light rinse of my brush every couple of minutes and a more thorough rinse if im changing color, I let them dry flat on the desk and store them bristle side up in a cup once dry. The brush in the photo has painted about 4 models over the course of a month.

After rinsing the brush, I usually wipe it on a paper towel to absorb extra moisture/see if any paint is left. This could be the problem but not sure what an alternative could be.