You see reports of distortion when capturing photos of fast-moving subjects using the electronic shutter. Here’s an excellent example of what happens.
This image was captured with the XF 56mm F/1.2 wide open, ISO 200 and a shutter speed of 1/10500 on an X-T1. Thos balls were in fact spherical.
So the next time I want to shoot family members playing backyard picnic games with very shallow depth of field, I’ll definitely look into a neutral density filter.