Only having Souls for Legos I have actually acquired appear at the Soul Merchant the reset after I've spent my Essences.
This has happened to me 4 different times.
I swear, I can hold on to my Essences for two weeks and never see a legendary soul that I can use. The minute I spend Essences on an Epic and the count drops below 20... bam!!! Legendary Soul for one of my Champions.
Another one that happened recently.
Needed 2 more Greater Magic Potions to complete the Dungeon mission to brew 10 Superior Potions. Set lvl 20 Magic Keep on auto for 20 runs. Came back an hour later... 12 Superior Pots, 6 brews, 30 something Mystery shards, 0 Greater Magic Pots...