correct way to use exportMaterialCollections

[willDTF]

I need to export USD from maya with material collection assigned to objects

reading usd-maya documentation, I need exportMaterialCollections and according to this ticket resolved, i dont need to setup materialCollectionsPath argument. so i tried :

cmds.mayaUSDExport( exportMaterialCollections=True, file=usdFilePath, )

this doesn't work, no collection are created. then :

cmds.mayaUSDExport( materialCollectionsPath='/Mtl', exportMaterialCollections=True, file=usdFilePath, )

same result. i tried many variations, without success. The only step forward i've made is with :

cmds.mayaUSDExport( exportCollectionBasedBindings=True, exportMaterialCollections=True, file=usdFilePath, )
material collections is created with the addition of exportCollectionBasedBindings, but no objects are in the collections

cmds.mayaUSDExport( exportCollectionBasedBindings=True, exportMaterialCollections=True, file=usdFilePath,materialCollectionsPath='/Mtl' )
gives the same result that the previous, collections exist but empty

How can I properly export a USD file with material collections assigned ?

thank you