Hi!
Thanks for the demo. Unfortunately, it seems to be a bug that was introduced in Highcharts v11. I've created an issue on GitHub, where you can track its' status:
https://github.com/highcharts/highcharts/issues/19093.
If you aren't obliged to use
shadow on your line charts, I would recommend disabling
shadow property until the fix is released. If you still intend to use shadows, you can do a fallback to v10.3.3.
First workaround: https://jsfiddle.net/BlackLabel/wmv37tnh/
Second workaround: https://jsfiddle.net/BlackLabel/xq25zaeg/
In case of any other questions, feel free to ask anytime.
Best regards!