Trainers worldwide have reported a weird bug with the last Premier Ball — it’s impossible to catch any Raid Boss with the last Premier ball.
There are problems with verifying/disputing this bug, but for safety reasons, play as if you have one less Premier Ball.
Niantic hasn’t given any official response to these reports, but the amount of reports is growing fast. Up to this date, no recorded reports of players actually catch a Raid Boss with the last ball were made.
It’s unclear what’s causing the issue, but the fact that only the last Premier Ball is affected is indicative of a desync happening between the Pokémon GO mobile client and Niantic’s servers.
Bugs like this are not unusual, especially in game play scenarios where the encounter ends after a fixed number of events occurs on the client (in this example, you run out of Premier Balls).
We’ve reached out to Niantic’s Support for further clarification.