I work in adtech and yeah if there is a discrepancy the advertiser will 100% contact their account manager about it. Same goes for the supply side, publishers maintain what they expect to be paid and will raise any discrepancies as well.
Usually it is just a misunderstanding of the metrics, but if there is an issue they will always open a ticket with the engineering teams. They also expect some level of discrepancy since the volume of data is so large.
