The Frontier Airlines World Mastercard connection shows the miles earned from card purchases (for example, 155 travel miles since the last statement). Frontier Miles are also earned by flying Frontier, and those miles only appear on Frontier's own site or app. Barclays never shows them, so the balance in MaxRewards is understated.

It would help to see a total Frontier Miles balance, either by connecting a Frontier account or by letting users enter that balance manually.