OK, I thought as much, i had the same problem many years ago, dog retreaved Hares to hand, single handed or with a mates dog, then ran it with a dog i didnt know once, yes once, that ate the catch, my dog started doing the same, i did plenty of retreave training and holding, letting him carry the catch for a while before i took it off him, it cured him, it took 2 or 3 weeks, i didnt run him with strange dogs after that and all remained well, in fact he was the best dog i,v ever had, hope you get it sorted.