That seems odd. A US bank would ordinarily send a check to the former customer's last known address if it had not been contacted by the former customer to make other arrangements. Barring that, it would hold the funds for a period of time (generally some number of years) as per the former customer's state of residence's unclaimed property law and then turn the funds over to the state. Chase does this pretty routinely.

Did you close the account or did Chase?

Probably best to send a short and clear letter to Chase advising that you have been advised by B6 that it issued a refund to your closed account and that your instructions are to send a check in that amount to you at a specified address. Leave out the rest as it is not relevant to the issue at hand.