Set the net clearance to 0.25 and mask clearance to 0.2 and you will get exactly what i show in my screenshot.
(For the screenshot by OP you will be able to set the difference to something much larger and still see the same effect)
The problem really is that kicad uses a different way to calculate net clearance compared to mask clearance. This means there will be issues. They however only show themselfes in certain conditions. (So large enough mask clearance compared to net clearance)