Many cards that were printed for paper (i.e. not originally from an Alchemy set) have been nerfed/buffed for Alchemy. This results in a new version of the card with the Alchemy set symbol. Such cards can be added to decks in Moxfield.
However, only the original printing of the card has been given an Alchemy version in Moxfield. For example, if you want to add the Alchemy version of Prosperous Thief to your deck, you can only use the original art. You cannot switch the printing to the Ninja Showcase art. In Arena, you can freely use whatever printings have been added to the client for Alchemy cards.
It would be nice to have access to Alchemy versions of all printings in Moxfield. This is just a cosmetic change, so perhaps it’s not that important. However, when quickly scanning one’s decklist, it is helpful to have the art match the art that one is familiar with.