@Evulse we also found that recurring charges are sometimes prorated when our customers are charged, but there might be a recent change about this.
If you look at this older post from the forum, someone quoted an older version of the Shopify documenation about recurring charges:
It should be noted that each shop can have only one recurring application charge per app. If you activate a new recurring application charge (i.e. service upgrade) for a shop that already has a recurring application charge in place, the existing recurring application charge will be cancelled and prorated for the duration it has been active for. Then, the new recurring application charge will activate. This means that upgrading and downgrading a user's recurring application charge or plan is straightforward; just change their plan and have them accept the new recurring charge.
However, you'll notice that if you go to that same page of the documentation now, the part of the sentence I highlighted that says the charge is prorated has disappeared.
Maybe someone from Shopify can clarify?