[...] some add-ons display buffs that aren't really there so it may be a fake indication.
i noticed this with my Srendarr too, i see Blood Spawn at my buffs even tho i don't use the set. Happens in dungeons too not just BGs. I also see rarely buffs like Burning Spell-Weave or some others i don't remember, even tho i don't use them.
It is true, that S‘rendarr shows the bloodspawn buff on you, if a teammatr wears it and it procs. But it is not tief to groupmembers actually, since you also may experience the buff icon, when you are not grouped at all. Same phenomen happens with the lich set. But it is only the icon showed as a buff, but you do not inherit the buff itself.
So nothing to worry, only some kind of UI error, the buff is not provided to you, only the icon is showing.