I was down to 9 XP, had a 100, 200, and 400 XP award I tried claiming each next to empty bars and they just vanished without credit. Also they did not show in my chest inbox like online AI suggested.
When you claim those XPs from daily quests or other sources they go straight to your XP bar - not inbox. There are also no known issues with them and haven't been for quite a long time so please do double check.