Export server seems to be broken when using Highcharts <= 10

[paulo-raca]

Starting (AFAICT) yesterday, the "Download PNG Image" feature is broken on many of my charts.

They should look like this:

But are being exported like this:

I put some effort in trying to figure out what was causing the issue, and this seems to affect many types of charts, but only on older Highcharts releases (<10).

Here is a minimum reproduction: https://jsfiddle.net/a3tedpu1/1/