Hey

It's not a bug.
Once the Serpent has been summoned. It's considered a "new" battle, once it's defeated, The team reapplies, passives/buffs/shields including standards and (Battle CD I believe). Particularly, people are finding ways to strategically use the summon to either absorb mysteries/refresh battle passives, etc.

I see.

I'll inform the team of the issue.
Thank you for reporting it.