which begs to differ, is this a known bug with the Spline function
I don't know, as I said I don't use the module, but you could look here (you can login with your forum credentials).
One thing that caught my eye though is this bug, which doesn't speak well of the implementation of the charts module, as the developer provided some lazy excuse (this is simply implemented by notifying the owner of the destruction).
Anyway, you could either file a bug report or try something else. My father (who uses a commercial license) gave up on the charts and is currently using QCustomPlot, so you could try that if it provides what you're after.
I hope that helps.