Just opened usd asset made in 20.5 in 21 and noticed that the normal maps looked weird in some places. They all had in common that they didn’t have any uvs and had 3d noises going into the normal map of the shader. I quickly added uvs to the objects and everything looked fine.
I can’t remember that this was an issue in Houdini 20.1?

Hi. It's hard to say without an example file, but normal maps typically do require UVs (especially if textured). If you're using height to normal VOP to apply procedural noise as bump, UVs aren't required since the tangent frame between height to normal and normal map nodes are consistent. If you're wiring 3d noise directly into normal map node, then discontinuity artifacts and other issues are expected.