I have found the same problem. With bigger jobs where I am reusing the same Element3D “scenes” or objects repeatedly from shot to shot, I tend to use TrueCompDuplicator to create distinct instances of each scene, and then use rd_CompSetter (these are fantastic scripts from AE-Scripts.com, btw) to extend or reduce the new duplicated comp’s duration to the duration necessary for the new shot I’m working on. I find that all of my lights will turn off at the original last frame of the comp I duplicated from, even though I’ve duplicated the original comp and extended the composition’s layers to be long enough for the new shot. I learned, through a painful process of trial and error (and probably discovered it accidentally while maniacally swinging my keyboard and mouse above my head) that the easiest solution is simply to drag all of your light layers to the end of the composition, so that their original first frames (heads) are now on the last frame of the comp (tail), and then extend the heads of the light layers you just moved back to the beginning of the comp. As another poster said, when you mess with duration and timeline position of lights, you get strange behavior, but this seems to reset them without having to recreate, reconfigure, and replace existing lights with all new ones.
Matthew Macar
DP, Motion Graphics, Editor
vimeo.com/mattmacar