Image with URL included in the exported chart only sporadically

## Unexpected behavior
When exporting a chart with a custom marker: https://jsfiddle.net/sartzberger/q4uxh98r/3/ it gets exported about 10% of the times.

The marker image is provided in proper, supported image url:
```
marker: {
    symbol: 'url(data:image/png;base64,iVBORw0KGgoA...
},

marker: {
    symbol: 'url(https://ssg.betterinvesting.org/public/markersales-b.png)'
}
```

