Straight to the top, eh? Ha, thanks.
Without getting into everything - traced the issue to a cancelled spa booking. It started adding the cost onto the bill, and then doubling it the first of every month since. I pulled all the dates of my payments, CC back-ups and confirmation emails from RC (just to make sure I wasn't mistaken).
For whatever reason, the cancelled spa cost has been subtracted from my initial deposit (!) weird, right? But again, have the CC records & RC emails / confirmations showing we paid the original, full amount.
I've definitely grown most frustrated with call center this week - they're clearly just reading what the system says, and not taking a look at what's happening. This despite the fact they can access the confirmation payments and see it doesn't add up.