Pokémon GO developer Niantic has announced that it will temporarily grant an extra Premier Ball in the Bonus Challenge due to a newly-discovered bug related to Raid Boss encounters. Read on below learn more about this known issue:
Last Premier Ball fails to capture the Raid Boss
We are actively working to resolve an issue where the Raid Boss always breaks free from the last Premier Ball available. To counteract the effects of this issue, Pokémon GO will temporarily grant an extra Premier Ball in the Bonus Challenge.
Gym Control bonus is not working as intended
During a Raid Boss encounter, Trainers should receive two bonus Premier Balls when they have a Pokémon assigned to a Gym that their team controls. We are aware that this functionality is not working as intended.
Source: Official Pokémon GO support page