hmmm, if those guitar and piano loops are ‘clownfucked’… meaning, not realistic or individually unique and originally recorded stereo images. i.e. artificially widened to sound MORE impressive, then they may change in volume between mono or stereo.
So its probably not consistent to start criticizing sound LCR mixing techniques based on that scenario.
Just record everything in MONO and pan it hard Left, Right or Center. Or don’t.
The whole stereo image ‘thing’ is based on a ‘phantom center’ anyway, (adding two speakers together will increase the perceived level of the loudest parts)… so is a compromise in essence to say the very least.