if you use „campaign contains XXX“ it might be that visitors returned to the page still having the (old) tracking code (if it hasn‘t expired yet).
another reason might be that a visitor returned to the page with the tracking code within a single visit timeframe (by default 30 mins). in this case adobe „thinks“ it‘s still the same visit and therefore the referrer is from the „first„ entry.
what you might look at is if a desired tracking code is set during a visit and see the referrers. to do this create a visit segment with a hit-container included where „tracking codes instances“ (metric) = 1 and „tracking code = XXX“
applying this segment shows only visits where the user had the desired server call. if you want, you might expand the hit condition within the visit container to only match on the first hit....