While I initially believed this to be related to MC-192718, this can also happen very close to the world spawn. Although generally not noticeable in normal worlds, by creating a superflat world with a quartz block surface and going to a village, sporadic generation of grass blocks can be seen outside village paths. Although not visible in the attached screenshot, this can also happen with dirt blocks.
Environment
Java 21.0.7
RTX 4090
Intel i9
32 GB RAM
Vanilla
The grass and dirt blocks are built into the structure files. The developers could replace the dirt and grass blocks with structure blocks? But this works fine. The only case you would even notice this is in alternative map generation, as you said. I would personally classify this as Working As Intended.