With /tree/children and the uri http://purl.uniprot.org/enzyme/1.1.1.- some of the child nodes do not have a prefLabel (or an _about) but only have a uri. The API docs imply that they should each have a prefLabel
prefLabel: "(-)-trans-carveol dehydrogenase"},
prefLabel: "3-methylmalate dehydrogenase"},
prefLabel: "D-xylulose reductase"},
This may be related but I found a mismatch between target class pharma count and the results you get when filters are applied:
the count is 17 but the actual results only have 15 items.