I think it should be showing that I lost the currency instead of gaining, but it would also be great if I could turn off the currency spending like with gold.

See recording: https://i.imgur.com/zfQ6EM7.mp4
Here with less "gain" due to fragment reducing cost: https://i.imgur.com/dZgNzDj.mp4

It's never meant to show fragment loss toasts. The module was last updated before Blizz added Enum.CurrencySource, so it still relied on values I put in there which by now are outdated.