Don’t know if it is a known bug or was reported before, but I have not found bugreport about it on any forum.
It works like that every time in 7.0.12:
No matter how much damage a shield received during the battle (unless it was destroyed), as soon as the Battle-End Screen appears all of it is forgotten – even the defenitely-damaged-almost-crumbling shield of a dead brother, which now is being listed among the loot as perfectly-repaired-to-full-durability, not to mention all the shields equipped on the rest of Bros when you check them in inventory screen right after the battle.