Had a bad surprise. "Clean MultiMaterial" utility from 3DS Max, when used on the whole scene, messes up all Corona Proxies multi materials.
Here's a list of symptoms I found:
-If the proxy has only a single material applied, it is fine.
-If the proxy has a "Multi/Sub-Object" material applied, then the utility removes all sub-materials except one (I didn't investigate which material it keeps, maybe the last one? idk)
3DS Max 2023
Corona 9