OK. But it's strange for the user.
Should be changed in the future.

Thank you

It's definitely an issue that should be looked at.
My workflow has been:
Change brush size > sculpt > undo

This both undoes the sculpting as well as the brush size change.
It might be a limitation of the current system, just want to bring some more attention to the bug.

The same happens in weight painting mode. So in my opinion there should be definitely a higher attention to this problem.

Just for reminder.

Still existing in v2.81 master.

Yes all settings brush must not have an 'UNDO', that's what I think.

In T67458#831567, @Malinguaggi (Voguart) wrote:

Yes all settings brush must not have an 'UNDO', that's what I think.

I know what you mean, but for the development this is not that easy as you think.
Basically we need always an undo, even when you set a value you need a way to undo the value.

The problem is in brush paint mode the user wants brush undos only, when painting or beeing "inside the viewport".
A way could be to create an undo system that is mouse cursor dependent. Depending of where the mouse cursor is, there the undo will do what you expect.

Examples:

1. If the mouse pointer is inside the viewport area, the brush painting or whatever you've done there will be undone.
2. If the mouse pointer is in a different area, like the sidebar, header or footer, then there the undo works. But this is tricky. Especially for the header inputs. The user moves the mouse automatically off the header e.g.

Not that easy....

Christoph Werner (Taros)

I understand that simple words or observations cannot write code or repair it and I am far from wanting to do it or pretend, I know that it is very delicate !!Now your example suits me perfectly.Maybe this problem will be resolved by 2020, of course I speak on behalf of the blender community.
thank you !!