I currently have 1 domain billed through WHMCS. It is in fact a domain that doesn't expire/renew until December 2020.
December 2020 is showing an Annual forecast of $20 (which is correct for the domain) and the Biennial of $200.00.
I checked in both the tblhosting and tbldomains database tables. tbldomains has one record, recurringamount of $20.00 with a next due date of December 2020. tblhosting has several records from all clients (obviously), but none of them have a next due date of December 2020. Also, none of them are biennial.
In tblHosting, I do have two one time payments from December 2018 of $100 each. That's the only combination of items that would total up to $200. These are both set to one time and they both have next due of 0000-00-00.