Text in exported chart is hidden when setting style in chart options

I want to report a bug when exporting a chart.
As soon as I start to set fontSize in style options the text is completely missing in the export file.

```javascript
chart: {
type: 'area',
zooming: {
type: 'x'
},
panning: true,
panKey: 'shift',
scrollablePlotArea: {
minWidth: 600
},
style: {
fontSize: '1.1em'
}
},
```

I created a fiddle to reproduce the issue:
https://jsfiddle.net/yrsv310n/2/

In browser the chart looks like this:


When exporting as png or jpg it looks like this:


Export in svg format works.