They have already said more than once that the quick ventures are TOTALLY random no matter what level your retainer is. MOSt of the time you come back with garbage, which means they get next to no experience. RARELY they come home with something nice and get some good exp for it.

Quick venture is a bad way to level your retainer. At higher levels your retainer comes home from the other ventures with better, and higher quality stuff consistently. Quick ventures are a gamble.

No bug here.