“For music with split out layers Drums is first (top) layer, Bass is second layer, Rhythm guitar or other percussion on three, then keyboards then solo instruments on bottom. This is customary”
Apart from live shows where audio is mixed live I’ve never known anybody have to mix their music for a TV programme or film. Usually get provided with or select a mixed stereo track Done by somebody else.
Probably my misunderstanding. But certainly quite unusual sounding for me.