Brush hardness at 0% makes edges/circles

It's called banding, and it's expected behaviour.

You are seeing banding because you are trying to create a gradation at the very end of the light-dark scale.

You have to remember, in 8-bit mode, you only have 256 shades of grey between 0% white and 100% black.

In the image you've created, you've essentially created a gradation that's about 80% Black at its lightest, to about 100% Black... that's only a 20% difference which means you have only about 50 values of grey to create it. YOU WILL SEE THESE BANDS and there will be 50 of them, give or take. This is not a bug, this is the limit of technology.

It's more noticeable in dark areas as our eyes can see the subtle change of shades more easily than at the light end.

Now, if you were to use the gradient tool, there is an option to "dither", which means they randomize the grey pixels a bit so that the banding will be less severe. You could also manually dither by using the Add Noise filter, but it depends on what you want to accomplish.

You could also work in 16-bit mode which increases the number of greys to 2048, that will create smoother transitions but you will have to convert back to 8-bit at some point, and then the issue returns.

I have the same issue. been bugging me. If photoshop wont fix it nor look at our community posts about it, then i'm cancelling my monthly subscription and moving elsewhere. Im not paying 35 euros a month for something i need but cant get.