In this fiddle:

You can see that the legend bullet for the first series is absent in the legend. By setting line 92 back to 'false', it works fine.

This disallows having a chart with several series initially set up but showing only one (or a few), letting the user choose which series to show interactively.

The documentation on Ext.chart.series.Series.hidden for ExtJS 6.5.1 seems incomplete, so maybe this feature was not fully implemented/tested if set at chart definition time?