The level of dissolved oxygen in the gelatin solution may seem significant, but the total amount added to the batch is going to be negligible. I've never had this problem with fining. My method is to introduce finings through the gas port with the PRV pulled, quickly close up and repurge the headspace, which may allow less exposure. But I'd still suspect the culprit source of oxygen was elsewhere. If bottled off the keg, that's a more significant opportunity for oxygen exposure. Beer guns and even CP fillers are not very good at purging the bottle and limiting exposure during the fill, so even capping on foam, the damage may already be done. Like Tommy asked, was the beer direct from the keg similarly oxidized?