gltf models are sometimes too dark

[srcejon]

What happened?

Sometimes gtlf models appear too dark. In this Sandcastle example, the same model is loaded twice (via different filenames), but one is darker than the other:

In this case, one model is specified via CZML using incrementallyLoadTextures: true and the other using incrementallyLoadTextures: false, as this seems to reproduce the problem most frequently. However, it can occur under other conditions as well and sometimes randomly (E.g. sometimes the model looks ok just rerunning the same code).

If we use the same value for incrementallyLoadTextures (either true or false) or use the same filename, then both models appear the same.

Reproduction steps

Bug can be seen in Sandcastle example. It doesn't appear 100% of the time, but 90%+ for me.

Sandcastle example

https://sdrangel.beniston.com/sdrangel/cesium/Cesium-1.124/Apps/Sandcastle/index.html#c=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

Environment

Browser: Edge, Chrome, Firefox
CesiumJS Version: 1.124
Operating System: Windows 11, Ubuntu 24