It's also for a rather unbelievable reason --- if your GPU is not "powerful enough", you don't get rounded corners by default.
You read that right: apparently rounded corners are so resource-intensive that if you don't have or disable GPU acceleration, they'll disappear.
As much as I absolutely hate rounded corners in general, it's astonishing the apparent inefficiency with which MS have implemented them. Then again, mediocrity seems to be par for the course with their developers: https://news.ycombinator.com/item?id=28743687
