Not sure whether its the same case as I am using MBB savings to schedule payment to MBB CC and payment has been 'reversed instantly'. What I noticed is, it was due to Cash Back credited after statement date.

e.g. Statement balance = 50.00
Cash Bach credited 5.00
Outstanding bal = 45.00

I scheduled payment I using statement balance 50.00, but due to the cash back the outstanding balance is 45.00 (by the due date) and I think MBB will not accept payment with credit balance. I have experienced this instance reversal 2x and if there is no cash back for that particular month there is no problem with the scheduled payment (no reversal).

Not sure if my assumption is correct. You may check yours.