Its crucial that your key doesnt upgrade this was also happening on beta.
1: 3 Chest a 2 Key
2: if it doesnt upgrade 3 chest it again you should have a 5 Key
3: Relog
4:??????????
5: Your 5 Key is now a 8 Key for a random different dungeon
easier translation thanks hazz
1. Complete a dungeon within the time limit for 3 chests
2. If you don't or the key doesn't upgrade by +3 repeat step one until you have a +5
3. Relog
Best part is you can keep doing this until relog and get even higher keys
for example +2 -> +2 -> +5 -> +4 -> +7 -> +16
+2 3chest > 3chest 2 again> becomes 5 deplete to 4 > chest to 7 > relog gains +9 for all the 3 chests