I think it will be hard to know why unless you can reproduce and leave in that state. How do you typically recover from this? A refresh or reopening the gallery?
I would also agree in that this seems like a minor issue. Has it ever happened within your actual application? Or was it only ever seen in the UI gallery? If it only happens sometimes in the UI gallery, it may not be worth chasing this. But if this is happening in your real world application, it may be worth pursuing, although it does seem more superficial than anything functional.
Just focusing on the field will resolve the issue. It is pretty reproducible using IE11 (not 100% but frequent).
Yes, since this is not a real world application, I don't think it is even worth debugging, to me. However they happened to pick UI gallery for regression testing and it is metrics to check if any changes to system are not impacting the platform.
FYI, the UI gallery section is just a collection of out-of-the-box dynamic layout and this issue itself should be reproducible.