Okay.. Still same.
Yep, and when we don't as in case like yours.. "Their system" can decide to simply disconnect the service. I think their billing software system is struggling to handle the creative billing and accounting mess they've created. Promo this, discount that, perpetual this, add and remove that... .
Unifi bills AFAIK has always been advance payment. When I signed up, got charged for the current month and the next month which include the next billing cycle. After that, it's always advance payment. For instance, from 13/12/2017 to 12/01/2018. Payment due date is 03/01/2018.
Better disturb them..