I agree that the XP rewards are inconsistent and should be changed. We were doing the Withermore's Jar quest, had shared the journal entry and done everything together. Both of us participated in the final conversation where the XP reward was given, but only the person who initiated the conversation and their controlled companion got XP. There is no good reason to do this purposefully so I hope it gets fixed.
Completely agree here. Generally people are playing as a team and are performing the quests together. Quests which arbitrarily only reward one participant are very off-putting.
This always leads to the point halfway through the island where my wife or brother ask me why I'm a higher level than them and proceed to interrogate me on where I'm hoarding all the extra XP.