My initial question got closed so I'm asking again because the answers in the referenced questions are inconclusive.
Consider these sequences:
A. Ally A Polymorphs a PC into a giant ape then ally B cast Light on the same PC. The logical result would seem to be a glowing giant ape. The alternative result of Polymorph being replaced with a glowing (human, elf ...) seems incorrect as it implies that polymorph can be dispelled with a cantrip.
B. Ally B cast Light on a PC first and then ally A Polymorphs the same PC into a giant ape. The same spells are cast except in different order. Is casting order relevant? If not then the end result is the same as in sequence A, a glowing giant ape. Otherwise casting order matters with the implication that Polymorph can dispel pre-existing conditions such as Fear or Charm. And that is a significant power boost for Polymorph.
Back to the Circle of Stars druid.
While in your starry form, you retain your game statistics, but your body becomes luminous; your joints glimmer like stars, and glowing lines connect them as on a star chart.
The starry form looks like a light spell although its a magical affect. Logically it should have the same Polymorph interaction as casting the Light cantrip.
I'm trying to understand the implications.
