In attempting to debug my previous issue (Minimizing cross-talk between main and embedded rendering? - #5 by viridia) I discovered a different problem: using a scene environment map in a scene with GLTF-loaded models produces some very strange behavior.
I wanted to experiment with turning down the intensity of the lights to simulate a night-time scene. What I noticed, however, is that only some materials were being affected by the change to the lights - that is, some meshes got very dark while others were hardly affected at all. It appears that the meshes that were not affected (or only slightly affected) by the lighting change all had this in common: they were all using materials that were loaded via the GLTF loader. Meshes that were not loaded via GLTF were affected by the lighting change. Meshes that had their materials replaced (that is, a mesh which was loaded via GLTF, but where the loaded material was substituted with one created by the program) were also affected by the reduction in light intensity.
Then I experimented with disabling the scene environment map. When I did that, now all of the materials in the scene were affected by the lighting change in a consistent way.
So, it appears that materials which were created by the GLTF loader are being lit differently than other materials. Any ideas as to why this could be?