Software: Unity 2021.3.2f1
While the model import settings > ‘Generate Lightmap UVs’ tool-tip says:
“Generate lightmap UVs into UV2“
The UV channel annotation in editor is 1-based rather than 0-based, so, in the shader graph, you may think you got access to your mesh’s original, uv1 channel, but it’s actually overridden by by a lightmap UV.