Hi
What is your measured pre regulator voltage?
Of course since the 'bypass' current will have some ripple associated with it, the point where this gets injected into the main 15 Volt rail may be a bit 'resistive', making it a 'layout' thing rather than the concept.
10uF is woefully small if there is no other R/C decoupling of the supplies for a mic amp.
Unless Thom or others from ART join in it is all speculation as to WHY this was done.
Many people make unfortunate mistakes!
Matt S