In Sonic Advance 2's Sound Test, the invincibility theme is named "UNRIVAL". What.
I think it's supposed to be short for "UNRIVALED." As in, having the invulnerability gives you unrivaled strength compared to the basic enemies, or something...
It also fits with the naming convention for previous invincibility tracks that had proper titles. The invincibility tune in Knuckles Chaotix was named Surging Power and in Sonic Adventure and Sonic Adventure 2 it was called (Still) Invincible...No Fear. Unrival fits right in with those.
Sonic Advance 2: How come that Dimps downgraded the graphics during the development? Just compare these screens: