What Ed describes is quite a common issue occurring with P3D rendering ground polys:
I assume the light splashes do not Z-test alpha?
In that case they don't render in front of any other object with a material other than basic solid. (in both of the sims)
As P3D seems to render ground polygons as normal objects this issue occurs while FSX uses the legacy "ground call" which just seems to work differently.
There are a few possible solutions:
- Use an extra version for P3D with alpha testing. P3D does a better job blending a material against itself so that edgy border look shouldn't occur
- SimObjects have better quality Z-buffering and therefor always blend where they actually are. So placing the lights with SODE can solve this. For performance reasons I'd recommend to merge them into one model for that.
- Attach an effect instead of the light polygon. This works for small airfields, because it's quite performance consuming it is not recommended for large airports.
Hope this helps a bit