Legend should contain series' className with a trailing space.
The trailing space in legend class is missing. With "className": "missing-space" this results into the class missing-spacehighcharts-series-0 for a legend item.
Fixed #5934, series.className was missing a trailing space in the leg…
@martijnpieters Thanks for reporting, this issue will be fixed in the next maintenance release v5.0.3. A possible workaround in the meantime is to add your own trailing space to className.
I was already using that workaround, however, thanks for the suggestion!