Mktavish might have been right after all, perhaps it was seriously crazy to replace the static lamps with pickable and placeable lamps that you can turn on and off. At least, I suddenly have this weird issue - post copied from the Nexus forum where I haven't gotten any response:

I have an odd issue with my mod which replaces the static lamps with havoked ones. In certain locations, the new lamps are simply not there. This is odd in several ways:

1. I never had this issue when making the mod and checking all cells. And I've never noticed it when playing the game more than a month for dozens of hours with my mod active, until yesterday.

2. When checking the cell in the render view in the GECK, everything looks as it should, i.e. the lamps are in place, but even if loading the esp dead last, the lamps are not there when entering the cell in the game.

3. It seems completely haphazard - it works perfectly in most places I've checked, but the lamps are gone in others. And they are set up exactly the same everywhere.

Really odd. I have no idea what might cause it - anyone who has an idea?

If I uncheck the mod, load a save, resave, check the mod and load that save - the lamps aren't there even then (in this case in the Underworld Chop Shop). But they are there when checking the Chop Shop with my mod loaded in the GECK. Completely weird.

Also, if I remove my mod, load a save and enter the Chop Shop, the vanilla lamps aren't there, so both the vanilla lamps and my replacers are gone from that cell no matter what.

Also, it's not an issue with archive invalidation. At least, it's not fixed by toggling it off and on in NMM.

Underworld Chop Shop with my mod in the GECK: http://i.imgur.com/jRHWv9a.jpg.

Underworld Chop Shop with my mod loaded last in game: http://i.imgur.com/66c0QKA.jpg.

What on earth can cause this?