A. if acc1 and acc2 is linked.
B. if acc1 and acc2 is getting 3x XP when killing something with only those 2 in party.
C. if acc1 and acc2 is NOT getting 3x XP when killing things with the lvl 80 main account booster in the party.

if A+B is true and C still happens. Then something is very wrong and it might be a bug.
Also make sure you are reading the combat text for the "3000xp gained blabla (+2000xp Refer-A-Friend bonus)".

because a lvl 80 booster does snitch alot of xp from you when you kill things.
and that could possibly be misstaken for a loss of RAF bonus if you arent reading combatlog for the RAF bonus text.

where are you killing things at the moment where acc1 and acc2 get RAF in there own party.
and where are you killing things where you do NOT get RAF bonus when booster is in party.
what mobs/zone .