There is a bug with group xp where - if you are too far from the leader of a group - you don't get any xp and the leader gets it all. If you two happened to be 'soloing' far apart, that could be part of the situation. I haven't heard of any other bugs recently.
FYI the way it is supposed to work, the xp only gets split down to ⅕ when you reach 5 members in the group. If there are 6, it becomes a 'bonus' as each person still gets 20% of the xp value of the mob.