With an external 4K monitor connected. Based on issues with the MX Master 2S.

When the mouse cursor is lagging or flickering on an external monitor

In my case, the issue was caused by an unexpected drop in refresh rate from 60Hz to 30Hz every time I changed the resolution—regardless of which resolution I picked.
The lag was terribly irritating and made it nearly impossible to work.

A refresh rate is the number of times your monitor updates with new images each second. For example, a 60Hz refresh rate means the display updates 60 times per second. A higher refresh rate results in a smoother picture.

To fix it, I downloaded the SwitchResX software and manually selected the correct resolution and refresh rate.

SwitchResX Settings Window

After switching back to 60Hz, the mouse began running smoothly again—just like before the update. 🙂