It's funny. While Apple can be commended for pushing forward with accelerated graphics for their user interfaces years ahead of their competition they've often been plagued with one form or another of odd frame buffer issues that result in these graphical glitches.
And using that term "glitch" would be more appropriate than a "bug" since it's a a momentary lapse in fluidity of the animation. It's honestly barely noticeable to the vast majority of the population and even then a trained eye would need to do an A/B comparison to see it in action. I am impressed that it is something you notice though.
If it interests you you'll find the same graphical glitches on MacOS dating back to OS 10's early days. The problem usually lies with the implementation of the frame buffer in the Quartz accelerated graphics code. This is basically a driver level issue and persists across various devices of specific make regardless of model. i.e. AMD fares better and expresses fewer glitches with Quartz animations on MacOS than Nvidia chips.
Now keep in mind this is a 2D acceleration issue and is often regarded as fairly low priority such as the performance is today. However if you check out the differences across iOS devices you'll see similar glitch disparities from model to model. And newer doesn't always equal better. Sucks but I only expect they will try to make t as smooth as they can, but this just may be as good as it gets due to caching and feamebuffer sizes in code or hardware.
