Generated lightmap UV's in OnAssetReimport

I’m trying to derive some data from lightmap UV’s in an OnAssetReimport callback. The callback itself fires and everything is valid, except when the mesh has a generated UV - which doesnt display.

The relevant code is below:

auto ObjectClass = InObject->GetClass();
if (ObjectClass == UStaticMesh::StaticClass())
	UStaticMesh* StaticMesh = Cast<UStaticMesh>(InObject);
	const float DesiredTexelDensity = 1.0f;
	auto NormalizedTexelDensity = 0.0f;

	if (StaticMesh->SourceModels.Num() > 0)
		FStaticMeshSourceModel& SourceModel = StaticMesh->SourceModels[0];
		auto LightmapIndex = StaticMesh->LightMapCoordinateIndex;
		FRawMesh RawMesh;

		auto TriangleIndices = RawMesh.WedgeIndices;
		auto LightmapTexcoords = RawMesh.WedgeTexCoords[LightmapIndex];
		auto Vertices = RawMesh.VertexPositions;

		if (LightmapTexcoords.Num() > 0)

If the asset has generated lightmap UV’s, the condition above is false, which is odd!