What happens to sessions left on a pass when it expires?
If you turn on "Forfeit unused sessions on expiry" (available from the Pro plan), then after the expiry date any unused sessions left on the pass are zeroed out and don't roll over to the next pass. The debt itself is kept — a new payment still pays it down.
If the toggle is off — and it's off by default — the remaining sessions stay on the pass even after it expires. When the nightly automation zeroes sessions you get a notification (from the Pro plan up), and the Settings → Automatic actions screen shows afterwards who lost how many sessions and when.