Weird post processing on blur default pipeline

Your scene is very dark, and as such precision is an issue and there is likely already banding. The reason enabling DOF increases this effect is that the filter would be working on the low precision areas multiple times over, thus increasing the intensity of the banding. On top of that, because it is a low-frequency filter the banding would be easier to see as the local contrast would be decreased. The post-processing stack should be applying dithering by default when any effect is active, although I have noticed that this often doesn't seem to work as intended. I can only suggest increasing the total brightness of your scene and dimming it in post through tonemapping/colour grading, or applying grain to mask it.