What is this random grey bar at the bottom of my screen – and why won't it go away?

HideyukiK​  - you are correct. The issue is with the Windows Spotlight image, not a faulty driver, monitor or GPU.  To fix it, I simply right clicked anywhere on the desktop and selected NEXT BACKGROUND in the dropdown menu. The background advanced to the next image and the gray bar was gone. 

I encountered exactly the same issue on two Windows 11 25H2 devices, and both devices were displaying the same Windows Spotlight mountain image shown in this post.

I located the cached Spotlight image here:

%LocalAppData%\Packages\MicrosoftWindows.Client.CBS_cw5n1h2txyewy\LocalCache\Microsoft\IrisService\

The affected file was named 134339913670106887.jpg. When I opened the JPEG directly, I found that the image file itself contained a large blank area at the bottom. This confirms that the gray bar was not caused by the monitor, GPU, display driver, or Windows desktop rendering. The image distributed through Windows Spotlight, or a resolution-specific version of it, was incomplete.

The issue disappeared automatically when Windows Spotlight changed to the next image.

I have reported the corrupted image to Microsoft through Feedback Hub:

https://aka.ms/AA13mmoq

If you experienced the same issue with this image, please open the Feedback Hub report and upvote it or select the option indicating that you have the same problem. This may help Microsoft identify the affected image and replace it in the Spotlight distribution service.

If the bar persists, it's likely a cached rendering issue. The most reliable fix is to restart the Windows Explorer process, which controls the desktop, taskbar, and wallpaper layer.

This is a strong possibility given your RTX 4050 and 13th Gen Intel setup. A known issue with Intel Panel Self Refresh technology (which saves power by letting the display stop refreshing when nothing changes) can cause horizontal lines or bars to appear at the bottom of laptop screens.

The photo shows the grey area above the taskbar, and your observation that application windows cover it normally makes the desktop background a sensible first test. It is not enough evidence to diagnose a failing GPU or display panel. Open Settings, Personalization, Background and temporarily select Solid color. If the strip disappears, switch back to Picture, choose a different image, and set Choose a fit to Fill. Microsoft documents that Fit and Center can leave uncovered areas when the image and screen dimensions differ. Also open the original wallpaper in Photos to check whether the strip is part of the image itself. If it remains with a solid background, capture it with Snipping Tool and compare that capture on another display. Report whether it appears inside application windows too; that distinction determines whether to investigate the desktop presentation or the wider display path.