You can use a 32 bit Float image.
What happens is that your brush has a smooth cosine-like shape. If you repeatedly apply that brush in the same place, the shape effectively becomes very steep and the jump in color becomes visible, because there are not enough colors. Now 32 bit is a lot more color information and it can handle those steep curves smoothly.
Forgive my horrible drawing, (my tablet is broken, so I even had to use my mouse...), but here you can hopefully see what I mean:
The smooth one is made with one click and a strength of 1.0, while the stepped one is made with a strength of 0.1 and many clicks (even more than 10).
Spikey is absolutely correct about 32 bit images. I went over this briefly in a livestream if you want to see that.