It seems that get_channel() is not working for any user channel above user7 (user8-15). As well as none of the "recently added" channels (like CoatColor, CoatRoughness, CoatOpacity, SheenColor, etc). It works fine for all the other "old" channels (like BaseColor, Height, user0-7).
using Adobe 3D Painter 7.4.1
I manually created user12 and user4 channel.
This not working (User12)
ValueError: There is no channel 'user12' in the texture set 'DefaultMaterial'.
C:/Program Files/Adobe/Adobe Substance 3D Painter/resources/python/modules\substance_painter\textureset.py(688): get_channel