We are aware of an issue whereby a negative balance is returned for your American Express credit card account instead of a positive balance.
Unfortunately, American Express do not provide us with a +ve or -ve value indicator for the balance via their Open Banking API, just the figure.
As this is a credit card account, we are therefore forced to assume that in the vast majority of cases the balance will be negative, but this means for the rare times where the balance should be positive, due to either an over-payment or a refunded purchase, the balance will also continue to incorrectly display as negative.
We're checking this with American Express to see if there is any way for them to provide us with a value indicator to correct this problem, but I'm afraid we don't yet have any timescales for a fix.