I've narrowed it down to the reference spline. If the spline has a 0 rotation on Z, then all is as it should be.
However, if I rotate the spline, by the amount I need, 31.881 in Z in this case, that is where the lights go missing
Attached is a max 2022 file showing the issue. You'll notice if you rotate the ref spline "Layer:3D Unit1 Interior Ceiling Grid001", back to 0 in Z, the missing lights come back.
Also, another weird thing, which has nothing to do with the rotation. You'll notice the bottom spline, generates the ceiling grid different to the middle and upper levels, even though its the same spline that was copied up and attached.