Could you provide the extension name that is missing.
Looking at the schema for geometry decal -
There is an “enabled” boolean property - but I don’t see one in the Material panel for decal.
But examining the gltf there is a mode extension with a value of “default” - this mode property is also not in the material panel.
There is a DECALMODE in the code also. Assuming this is for the mode extension.
RENDERUNDERCLEARCOAT (ext name underClearcoat) is in the mix too, but I see no corresponding gltf extension property