NSDecimal addition generates large number on iOS 18

[kbot]

Our production application (linked with iOS 17) has been seeing reports by users updating to iOS 18 that previously valid decimal account balances are now huge numbers.

Our code for calculating the balance:

NSDecimal totalBalanceDec = {0};
    NSArray* arrBalances = (NSArray*)JSON_NULL_SAFE([responseDict objectForKey:@"balances"]);
    NSString* currencyType;
    for (NSDictionary* balance in arrBalances) {
        currencyType = JSON_NULL_SAFE([balance objectForKey:key]);
        if (currencyType && [currencyType isEqualToString:value]) {
            if ([balance objectForKey:@"value"]) {
                NSDecimal decCopy;
                NSDecimalCopy(&decCopy, &totalBalanceDec);
                NSDecimal dec2 = [[balance objectForKey:@"value"] decimalValue];
                NSDecimalAdd(&totalBalanceDec, &decCopy, &dec2, NSRoundPlain);
            }
        }
    }
    return [NSDecimalNumber decimalNumberWithDecimal:totalBalanceDec];

Sample balances response data from our backend:

balances =     (
                {
            balanceType = CREDIT;
            createdDate = 1633544314000;
            creditType = INCENTIVIZED;
            currencyType = USD;
            id = ff8081817c55441b017c56d41d5811f7;
            lastModifiedDate = 1633544314000;
            value = 0;
        },
                {
            balanceType = CREDIT;
            createdDate = 1634856010000;
            creditType = PAID;
            currencyType = USD;
            id = ff8081817ca48459017ca502ffba0288;
            lastModifiedDate = 1726768624000;
            value = "75.2";
        },
                {
            balanceType = CREDIT;
            createdDate = 1637278758000;
            creditType = EARNED;
            currencyType = USD;
            id = ff8081817d00a009017d356b33ae7c58;
            lastModifiedDate = 1725407407000;
            value = "2.22";
        }
    )

Previous to ios 18 this would have produced 77.42 but after updating to iOS 18 it looks like this: