If it has a spring detent notch, and the spring is engaged, how does it walk? Is the spring itself moving a bit, allowing the pin to move?
I would NOT modify the frame in any way. If the spring tab is indeed flexing, allowing the pin to walk, perhaps you can have a new pin made, which is slightly longer, or has the spring detent moved a bit. Moved to the point where after the spring is loaded, and does it's job of preventing further movement, the pin is still well inside the frame. Maybe the spring detent notch simply needs to be a bit deeper. You can probably do this yourself with a very sharp,thin file while spinning the pin in a drill ( using the drill as a makeshift lathe).
I don't like oblonging the pin, as it will eventually effect the hole in the frame. I am also wondering if there is a correctly sized pin that has threaded holes in the end ( like available for the AR) that may work. But ultimately, it should work as designed.
As usual, pictures will help us all diagnose and provide possible fixes. There are many clever people on this site, and they have helped me every time.
DON'T MOD THE FRAME!!!!
Good luck
Rich