In the newest snapshot, the menu background dirt texture has been replaced by a darkened background. And the dirt texture has been moved to the build-in Programmer Art Resource Pack. The background in Pause Menu was a darkened background in Minecraft 1.13.2. But when I changed the resource pack to Programmer Art in 24w09a, the background in Pause Menu also changed to dirt texture.

The picture shows the differences between the two versions.