So I've tested this again. With a simple scene. It doesn't work if you work with Xref Objects. The pivot points are where they should be.
The object paint is a build in functionality in 3ds max to paint objects on other objects. Reason I was using this is because I 'm working with surfaces that are above eachother. Forest pack doesn't allow placing of objects on surfaces that occupy the same UV space. I don't want to split them up and don't want to unwrap or similar. It's easier to place them with object paint and convert them to forest pack afterwards. But apparently the instances to FP script doesn't work with Xref Objects (probably because pivot points of Xref objects are relative to the scene's origin where they originate from or something).