GG I run into the same problem yesterday* for the first time. Card payment was declined, because of low funds, were enough money was on the backup-Account (which steps in, last year, without a problem) but did no longer. Support says, the backup feature does not exist...
But it is also shown on there website: https://bunq.com/benefits/secondary-pin
I would guess it is a BUG and they are doing something big with the cards??
*https://together.bunq.com/d/6258-secondary-backup-account-with-apple-pay/26