Often it can be easier to reproduce and fix a bug if we can see it happening. This is easily achieved with a screencast.
Starting from iOS 11, you can now record your screen on iOS devices. Please see the helpful guide provided by Apple at How to record the screen on your iPhone, iPad, or iPod touch.
To keep the upload/download size of the resulting video file low, please ensure the video is as short as possible while still capturing the essence of the problem you’re reporting. Once recorded, please send the screencast with a description of the problem to us.