Why does the recorded screen video flicker when I record it with my phone?

This is normal behavior when the DyAc  function, which uses blacklight control technology to reduce the motion blur of the monitor, is enabled. If you can see flickering or moving scanning lines: it comes from the camera sensor and the monitor having a different scanning frequency.

The line direction could be horizontal or vertical, this is typically related to the camera direction (placing the camera vertically or horizontally) or how the camera sensor behavior is designed.