Hi there, thank you for reporting that.
It seems that it is not accessible indeed, which might be a bug or lack of some features.
Could you please report it on our GitHub Issues page? Our Accessibility team will take a deeper look into that, so you can wait for a fix/workaround there.
GitHub Issues: https://github.com/highcharts/highcharts/issues
We are sorry for the inconvenience,
Best regards!