Maybe it's your solving style?
In around 10'000 solves with different cubes I had 2 corner twists.
One with a GAN i3 smartcube, the other I can't remember.
But in both cases I realised it the moment it twisted and it was in both cases my fault
I almost never corner twist with my RS3M, it happens significantly more often when I do 2x2 and still not more than once every 100ish solves. You may have the cube set up too loose