HS and stacking buffs.

That probably happened because you had someone between level 105 and 109 in your party. You have to have a certain total level of leech-eligible characters in your party if you want 8x and HS to stack for your own kills. I think your level has to be 1/3 or 1/6 or something of the sum of the levels of everyone in the party, or somewhere in between. Welcome back rings aren't affected at all by this.