I could be wrong but I don't think you can create a custom brush with hardness control.
thank you for your answer
I spent a few months before Photoshop CC2017, under the Windows 7 system have not seen a similar problem.
Your software is likely to be caused by some file is missing, suggest the new installation give it a try.
I noticed this as well a few weeks ago and found absolutely nothing online to help. I think the problem is the Adobe built in brushes can be dynamically feathered which causes softness, whereas 'custom brushes' are based on a fixed brush preset that can only be resized and transformed. You can make Photoshop soften custom brushes, but it's tedious and you really need a good reason to do it. There is no point doing this with round brushes as they all dynamically soften, but as an example this is how you can soften.
Below is stroke made with brush 30 which has 100% hardness-for now just imagine this is a custom brush. Note for this to work you must paint on to a transparent layer.
control or command click the layer thumbnail to select the stroke and go into 'Quick mask' mode-Q on the keyboard.
Apply a Gaussian Blur. With this method you can visually see the feathering happening, whereas with the 'feather' command it's trial and error.
Exit Quick Mask mode and create a new blank layer-the feathered selection will still be active. Now fill the selection with black. Alt or Option backspace on the keyboard. You also need to turn off the original 100% hardness stroke on the layer below. You should now have the custom brush softened.
Finally Define a new Brush Preset and you have a softened 'Custom Brush' This can actually be useful with hair and fur brushes.
I would imagine this could all be scripted and you could then just dial in a feather/Gaussian Blur variable, but personally don't have a clue how to do that