I have set the color to brightest yellow with the emission strength as 50 as you directed
Then open the model in https://gltf.report/ and open the metadata tab.
You should see
KHR_materials_emissive_strengthlisted as an extension – if not then your version of Blender is too old.
I did check it in the website and it does have that extenstion
Only if your spotlights are so bright that they cause a surface to reflect more light than the luminance threshold. That is “as intended”, as you might very well want to see bloom off of the strongest reflections, or not, you can choose the threshold and emissive values accordingly.
I halved the brightness of all my spotlight for such case so as to avoid the entire issue of creating strong reflections, it did improve a bit as i can see some of the white marble texture.
I did try to tinker with the threshold and intensity, I dont see any expected difference
is there a blog post or something i can read to get better at this ??