Good Afternoon @Magnus,
Thank you for contacting us with your question!
This is not that clear but we have two ways to overcome this:
1) You can config the SourceDataPointList records tooltips or fill the SourceDataPointList[n].Tooltip with: "Test header <br> test pointFormat"
2) On HighchartsJSON fill it with:
tooltip: {formatter: undefined, headerFormat:'Test header<br>',pointFormat:'test pointFormat'}
This formatter function is used by our implementation to customize the charts experience, if we need to override tooltips we need to block the function;
Please let me know if this helps.
João Ferreira