That's no glitch and it's not the biggest arc bonus you will get in the Croatoa trial, but you must succeed at every mission to get that bonus. Fail to stop the stampeding Fir Bolgs and it gets a lot smaller.
Hint: When you get the "30 Firbolg" mission...use the mission drop feature. Yeah, you can only use it once a week, but that one is TOTALLY worth dropping.
