Customer created several subscriptions using the same promotion.
Per Customer Limit (the maximum number of subscriptions that a single customer can get at promotion terms) is set to 1 in the promotion settings.
The issue is caused by the bug with id #PBA-55800 ("A promotion may be applied more often than allowed").
Per Customer Limit counts the quantity of active customer's subscriptions. After canceling the first subscription purchased with promotion, customer can purchase another one with the same promotion.
Update to PBA 5.5.6 or above, an issue is not reproducible there.