The JSON reports allow you to use an annotated parameter to get an easier to workw with format. This appears to be broken in 18.11 and possibly higher up versions: /cgi-bin/koha/svc/report?name=name_of_report&annotated=1 annotated 1 and 0 give the same format. See bug 11491 for initial implementation. It works fine in 17.11.11 If you have scripts building on this, it's quite a problem.
Resolving - I think cache or something confused me, works fine now!