I don't know if this is a bug. But when I reset having only the book and staff equipped, with no items in inventory, it disapear until I buy a item or gem.
I did some testing, if I buy a equipment or gem, the amount of souls that I spend it's the amount of souls that I can get if I do a reset. I guess this is a bug. Sorry for the bad english.
woow nice idea, (also in the shop) well i'm gonna add this GEM to the list :) cheers