Revit API 2025 NewFamilyInstance always with default value

I am new to a project where we work with the Revit API to import IFC luminaires in Revit families. I am running into an issue when creating a new FamilyInstance using:
public unsafe Autodesk.Revit.DB.FamilyInstance NewFamilyInstance( Autodesk.Revit.DB.XYZ location,Autodesk.Revit.DB.FamilySymbol symbol, Autodesk.Revit.DB.Structure.StructuralType structuralType)

the Z position of the created instance is always "4,9212598425196781" feet higher than the values that I manually calculate from the IFC values (which are given in millimeters)

I convert the IFC values using: code ConvertToInternalUnits(double value, ForgeTypeId unitTypeId) Has anyone experienced a similar offset issue or knows what might cause this constant Z-shift.

Any guidance is appreciated. Thank you!