OK thanks @Guillaume_Pelletier I guessed this would be the case.
Instead I’m trying to do something a little different. If I can figure out how to have the same object, at differing levels of opacity, I can crudely swap out the models, to make it appear as if something is fading.
I can get the desired effect in Max, but no matter how I export or tweak the materials I just can’t get the GLB file to match. The way I’ve done it in Max is to use 3 different files for transparency (first being 100% black and white, 2nd and 3rd getting lighter / less black).
In the material, I’m using a black material, using the cutoff transparency mode.