Will they? Who knows. Should they (and probably will they?), no. I think they will be fine with it. The rule is a throwback to when you were only permitted three stops per continent. The rationale being that the end of the trip was the third "stop" that you were allowed in the initial continent. Just point out that it's not a stop and that you are ending your ticket there and they will probably not say too much. They will probably question more your ability to stop anywhere in South America.