iPhone volume change also changes volume of 1st zone in group

Continuing the discussion from Confused about grouped volume controls:

I’ve found an issue where changing the volume of an iPhone using Device Volume causes the volume of the 1st zone in a group to also change in a very confusing way.

The experiments I’ve carried out to investigate the problem are in the linked post.

Could somebody take a look and tell me if this is a bug or a ‘feature’?