Hi Alik,
Greetings from Syncfusion. We have analyzed the reported scenario and we would like to let you know that, we already have the issue and it has been fixed already. With our latest package(v18.1.45), the reported issue will not occur. We have created a simple sample using the spline area series with border mode set to top and with the first data point value given as null in the series but, the reported issue is not replicated.
The sample for reference can be found below
If you are using the older version of our chart widget, kindly upgrade to the latest version from below.
If you still face any issues with our updated package, kindly try to replicate your scenario with the attached sample and revert us. This will be helpful in providing the solution sooner.
Thanks,
Sriram Kiran.