I know there is an official bug with the current mapping (naming) of exported GLB but I may have found a new one.
If in the editor, some mesh don’t have any material like this:
The exported mapping will ignore them and shifted everything. So in this case, the material on meshInstance 17 will be applied on meshInstance 15, also causing the last two meshInstance to be associated with a default material (because everything is shifted).
Here’s the content of the mapping file: