Well… it also comes down to approach…
The Fill area is etched, so you do not really want a sharp corner there, plus that corner exceeds the clearance so it is excessively conservative. That makes a radius logical.
The Mask area is usually simply a flashed larger pad, so until there was a rounded corner pad, making a rounded corner mask would have been hard to do. Which explains the square corners…
Because the mask-allowance is usually rather smaller than the fill clearance, this subtle corner effect is not usually noticed.
Here, we usually set the fill clearance a notch above the PCB quoted rules minimum, otherwise you create a whole-board expanse of minimums, which is tougher on the PCB FAB, than ‘a few traces pushing clearances’.