There seems to be an issue, believed to be Extension:Kartographer producing "Couldn't parse JSON" errors when SPARQL includes queryhints, described at https://www.wikidata.org/w/index.php?title=Wikidata:Request_a_query&oldid=867570503#Optimizing_a_query_for_the_mapframe_extension
I'm afraid it's all a bit over my head. Grateful for some knowledgeable input.