I am doing some testing with USD at the moment and it seems that UE does only recognize the first primvar:st uv set and nothing else. There are also some issues when the primary uvset is called different than st. E.g. Blender has a default name as UVMap and therefore no UVs are available inside UE.
The primvars are there in the stage viewer in UE but not showing up on the static mesh in the UV map drop down.
Is that currently in the USD implementation not supported?