Mei’s Coulder power usually rolls her forward wherever your camera is facing. However, if you are moving your camera quickly while you cast the ability, it can start rolling in a completely different direction, even fully backwards to where your camera is facing.
Once you know how, this is VERY replicable. Load into practice range, pick Coulder, and flick your mouse to the left or right as you cast the ability. I also included a video example of the bug in action: Mei Coulder Bug
My best guess is that when you use the ability, your camera stops rotating before your model stops rotating, and the ball rolling uses the model’s orientation for its chosen direction.
Side note: Would be very nice if we could have the option to control the ball like Wrecking Ball’s movement, as a hero-specific setting? But either way, this is a really annoying bug and happens at least once a match on Mei.