When completing a quest, if the quest had a portrait frame when receiving the quest, this frame will be shown while the default UI doesn't.
This is due to Storyline using GetQuestPortraitGiver instead of GetQuestPortraitTurnIn when turning the quest in.
Note : As of right now, GetQuestPortraitTurnIn doesn't return the mountID while GetQuestPortraitGiver does.

Also Blizz's usage of QuestFrame_ShowQuestPortrait on quest turn-in is borked atm