Sometimes when I kill bigfoot with my friend Cassidy or Odin, the bf not give me exp. Even in one I was soloing, after taking 70% of the hp, I called Cassidy to help in the last 30%, but received only 6m exp (even with hs). I tried to give several hits before and after going fm receive HS, but it really is random, sometimes I get 12m~13m exp, others only 100k ~ 800k. The last we killed (about half an hour ago), is in the prints. Thank you.
If you change the party while killing Bigfoots this can happen, everything just gets screwy. You need to make that whoever you invite to the party for the leech exp (or as an attacker) is invited before the Bigfoot has started. I've had this happen to me as well and also had leechers receive far less experience due to this.