Diagnosis
You are observing faint horizontal banding, often called "scan lines." This is a known characteristic of the M27Q X's Fast IPS panel and its specific subpixel layout, not a defect.
How to Fix It
This is inherent to the panel technology. You can minimize its visibility. In the OSD, navigate to Picture > Sharpness. Reduce the Sharpness setting from the default. A setting between 0 and 3 often reduces the prominence of the fine line effect.
If That Didn't Work
The effect is most visible at specific refresh rates and color depths. Try changing the refresh rate from 240Hz to 144Hz via Windows display settings or GPU control panel. Alternatively, change the color output from 10-bit to 8-bit in your GPU settings.
Heads Up
This is not the same as dead pixels or image retention. The lines are uniform across the screen and are a static property of the display. It is generally not grounds for an RMA unless accompanied by other faults.