I'm also using the UI mod and haven't had any issues so far.Well something is definitely wrong then. I've cleared normal chaos with the chaos choice on Aloith mission. Then I did very hard law choice on Alioth mission. Since I did Alioth mission on very hard, I should have unlocked a whole bunch. I only got Sheila, Theofrad, Leona, and Fana. Everything else doesn't show up. I left some units in the key pool, so I can do ike 3 pulls on day or night for 10x. I get the usual fodder and then it runs out before I get any of the unlocks.
In case it was a low% chance and I had to pull the unlocks before running out of normal units, I reloaded like 50 times and did my 4 pulls. It never shows up still.
I am using the UI mod if that somehow affects this thing....
If you got the message that you unlocked the characters for the key of destiny pool, in this case Youthful Loki and Hain, you should be able to draw them, if you can't, i don't have a clue what it could be.