Looks like it was precision inconsistency between frontend and backend calculations. JavaScript is trying to represent 19.99 with as much precision as possible. Since computers don’t have infinite precision, it picks the number that is closest: represents 19.99 as 19.98999999999999. So after the value is processed on backend, multiplied by 100 (Stripe API requires amount to be passed in cents) and gets fractional part removed, it becomes 1998.
This will be fixed in the upcoming version of the Subscriptions addon. Note you might have to re-create your plans if they still show improper amount.
Thank you for reporting this.