Hi, good question.
The Mana is considered in a pool in the game, a resource, the starting item and familiar mana are just a starting point for the mana pool of your character. What's important overall in that mana pool, so the item wouldn't loose its charge when used. Basically, as long as the mana pool has mana still in it, that mana can be converted into any spell, item use or familiar help.
When I created the game I didn't think about this detail from a narrative point of view, it was a very interesting question, maybe I will add something about it in a future update to make things more interesting for the story being told, thank you for pointing it out :)