Additional Info: Even if there’s only one material in the layer stack, the material name will NOT be used as prefix. InstaMAT Studio exports just the output names as suffix.
Used this Image File Format Extension: {LAYER.STACKNAME}_{OUTPUT.NAME}
I was not able to reproduce this. I exported a single material section from a multi-material asset texturing project, and the layer stack name was included in the output file names using the same file name format you had entered.