Hi,
Thanks for the question!
It is not intended for lines to go missing after exporting. This is a regression which appeared with the latest release, Highcharts 11.3.0.
I see that you've already reported this problem on GitHub here:
https://github.com/highcharts/highcharts/issues/20602 so our developers will take it from there.
In the meantime, you could roll back to Highcharts 11.2.0, where this issue does not appear:
https://jsfiddle.net/BlackLabel/3fyjL2eh/
I know this might not be perfect, but please let me know if you can currently work with that.
Best regards!