We’ve periodically had reports like this, but so far haven’t been able to reproduce the problem. See here:
If you can give us step-by-step instructions for reproducing the problem - preferably by starting with the Cesium for Unreal Samples project - we can investigate.
The most likely cause is that Cesium is using the “wrong” camera to select tiles to render your movie. But it’s not clear when and why that happens.