I’ve heard two schools of thought about that. The bow light “circle” is a transparent piece of plexiglass that has apparently always been there but was once so covered over with dirt or paint that light didn’t shine out of it very well and you could barely notice it on tv.
Some say that it was always MEANT to be lit up for viewers to see but just got too dirty for anyone to realize that it was there in the original tv footage, and others say that It was NEVER meant to be lit up/noticeable and was actually just an “access hatch” to replace the lightbulb that illuminated the three deflector array portholes on the nose of the Enterprise, and that it was intentionally covered with dirt & paint so that no one could see it.
In the end the conservators during one of the prior restorations decided to clean the clear circle off and make it transparent again.
TL;DR: It’s always been a part of the 11’ filming model but I don’t think anybody really knows for sure if it was ever actually meant to be seen on television.