className does not seem supported for series.gantt.data in Highcharts Gantt (https://api.highcharts.com/gantt/series.gantt.data). It is usable on initial load, but not for updates. This makes it impossible to use in styled mode.

A Stackoverflow question with jsfiddles demonstrates the issue and possible workarounds.

https://stackoverflow.com/questions/61088143/changing-point-classname-in-highcharts-gantt/