It seems that, in an effort to make the airports FPS friendly, FSdreamteam and Taxi2Gates in KSEA have configured these light fixtures to do just that--disappear from view--, under some circumstances and depending on the view zoom.
If you said the problem happened ONLY at wide zoom settings, it would have been easier to reply earlier. You said it ONLY happened with the PMDG 737, which was a bit misleading...
In any case, yes, we obviously use LOD, and any PROPERLY optimized scenery should try to use LOD when possible, to save fps. ALL our sceneries use LODs, which mean ALL of them will have detail disappear automatically after a certain distance AND when using unrealistically wide zoom settings.
It has been universally acknowledged the most realistic zoom setting in FSX is around 0.70. Which is why, we try to ensure the scenery will always be visible at that zoom setting. This doesn't mean the objects will disappear as soon as you go wider than that. They usually don't, and most of the times you can go as wide as 0.50 or 0.40. It only means we don't test the scenery in ALL places and view points to be always be visible at zoom settings wider than 0.70.
Still I think that for my particular machine capability I would rather not have that feature, is there a way you can make a patch to let the fixtures stay put no matter what ? apparently some taxiway textures are also affected.
The LOD levels are correct and we designed the scenery to work well on most machines so no, there's nothing to "fix" or "patch", because the scenery is performing as designed.
To be more clear: it's way EASIER to not optimize the scenery, and just get rid of LOD levels, and have "everything in". This is what lazy developers do, because they work less (an object without LOD takes WAY less effort to create, export and test), and you pair dearly with fps and VAS loss.
It's possible that, thanks to our extremely flexible scripting engine, we might add a feature in the future to control LOD levels "on the fly" (yes, we can do that without annoying configuration utilities that forces you to restart FSX and reinstall parts of the scenery), so you will have the freedom to lower your fps and increase your VAS usage as much as you want, but at this time there's no such feature yet.
We'll try to have a look at it when we'll have some spare time.