I've sent a support request, thanks Genreve. I hope they can help.
The payment to supplier just says "Transfer from deposits" which I understand, but the problem is that there's no payment in the "Deposits with vendors" account to offset that amount, which is why there's a credit balance at the end.
It gets curiouser and curiouser! If I generate a remittance advice from the "Payment to supplier" window, the phantom bill disappears and the correct figures are there. It seems that MYOB knows, deep inside its soul, that it has done the wrong thing! :-D