I have no idea of the code inside the sim but, obviously, I would expect FSX should be able to deal with an FSX-native file, and the KLAX FSDT certainly is. But again, haven't you already tested you have the issue even with FSDT KLAX removed ?
Perhaps you just discovered a bug in the old FSX not being able to handle properly Backcourse ILS. I wouldn't be surprised of this, FSX:SE fixed many small bugs like that, and if you use FSX-SP2 (instead of FSX+Acceleration) there are far more serious bugs in the rendering left there, which of course nobody will ever fix.
Surely not a reason enough for us to create a purposely inaccurate scenery that surely works with the latest simulators, just to side step possible bugs of a 13 old sim.