Yes it is a problem. Working in 16 bit will help, but if you are painting in layer masks, then they are 8 bit regardless of the image bit depth. Adding some noise (jitter) also helps, but that always feels like a poor solution to me when you are trying to get a super fine gradient. I also tend to use Gaussian blur rather than a brush for large soft gradients. Use a new layer, and make it a Smart object so you can go back in and fine tune.
But 16 bit will be the best way to go, and the slightest bit of noise. Here again, with a Smart Object layer you can fine tune. I like to use Camera RAW > fx > Grain.
This example has the original on the left, and Gaussian blur (150) plus fx > Grain (5, 20, 20) on the right.
