With external 4K monitor connected. Based on the issues with MX Master 2S.
When mouse cursor is lagging or flickering on external monitor.
In my case, the reason was an unexpected drop of the refresh rate from 60Hz to 30Hz every time when I wanted to change the resolution, no matter which.
The lag was terribly irritating, it became impossible to work with this issue.
A refresh rate is the number of times your monitor updates with new images each second. For example, a 60 Hz refresh rate means the display updates 60 times per second. A higher refresh rate results in a smoother picture.
To fix that, I had to download the SwitchResX software and select the proper resolution manually.
After switching back to 60 Hz, the mouse started running smoothly as before the update. 🙂