The pixel engine's airbrush parameter doesn't work with the Intuos 4 tablet on Windows 10.
This is an old windows only bug that dmitry wanted to solve by making airbrush time-based somehow. might be a nice google summer of code student job if we want to test people's mettle.
I am guessing that this is fixed in this commit: commit 88f88fe7a432a0202db8dcba22f6b5f2e2d4868e Author: Boudewijn Rempt <boud@valdyas.org> Date: Fri Oct 7 11:16:05 2016 +0200 BUG: 361293 Fix Wacom Airbrush wheel Differential Revision: https://phabricator.kde.org/D2950
That is not a fix for this. This relates to the airbrush option in, amongst others, the pixel brush. It doesn't work properly on windows.
This is fixed in master, and I think 3.2, so I will close this. If you can reproduce this with the rewritten airbrush option, please do not hesitate to reopen this.