A Cargo API URL like the following will work fine:
...but a URL like this will only display blank data:
...and so, for that matter, would a URL like this:
I looked into this, and it's happening because MediaWiki's API code assumes that any data field whose name starts with a "_" is actually metadata. (See ApiResult::isMetadataKey().)
There may be some kind of hack around this, but I think the best solution is just to display an error message if any of the field aliases (or field values, if no alias was set) start with a "_".