Touchy? The natural assumption for a license activation issue is just that, an issue with the activation process. It would explain why some payware aircraft systems work and some do not. In my own experience, I was never able to properly activate a particular Flight1 payware aircraft, even with support. Every system worked except main gauges and some lights and ultimately I declined ownership.
There is nothing about a scenery that interacts with the render of user aircraft lights on its surface. It is a software incompatibility, caused by combining products from different compilers into the same render scene. Essentially, the render is glitching and it is not necessarily true that the same aspect of a FSX compiled scenery that prevents dynamic lights from showing on a ground polygon, also prevents them from showing on a user aircraft. To establish this, you would have to render the scenery without the ground polygon, ascertain that dynamic lights do reflect off the remaining default ground and that they also reflect off the user aircraft. Until this check has been performed, all that can be said is that something about an FSX scenery, still apparently, because it could still be something else, causes the PMDG 77L for P3Dv4 dynamic lights to not work within approximately 3000 feet of the named scenery products.
The solution would be just as practical to replace all dynamic lights with volumetric ones, as it would be to recompile the scenery to v4 standard, or you could just maybe wait for the update. I have it on good authority that Changi is in the works...