If you find a bug on the bounty campaign will you use it to gain more coins or email the brand?
Also, it is not said by the brand that rewards will be given for finding the bug. Will they still give reward if you email them about the bug?
You should discuss this with the brand or the campaign manager. Each and every one makes his own decisions about rewarding.
I would notify them whatsoever, since exploiting bugs to earn money is actually stealing. Or course, I would be somewhat disappointed if I'd get no reward at all for that.
Its not actually stealing yet you are the ones finding it out and you do exert effort on finding those bugs and its just normal for them or on the team to compensate on what you have done. Even I, would definitely be disappointed when you do find a bug specially major ones and you havent given any reward.
On previous years i do see some good number of bug bounty but nowadays i rarely see those things already.