I did some testing in the debug room and I can confirm that letting Bubble expire during the fight will have identical results to ending the battle while it's active. That means Keine will have 1 HP should Bubble run out during the fight.
After confirming that, I tested Bubble + running away from combat on a few characters:
- Reimu, Marisa, Youmu, Remilia, Patchouli, Utsuho, Koishi: normal HP after fight
- Meiling, Sakuya, Bubble user: +50% HP after fight
Is there anything different about those 3 compared to the others?