Why is Glo predicting my AF date a day behind?

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Hoping someone can help me figure out what I'm missing...
My cycle has been a consistent 26 days for the past 5 months. Like clock work, AF shows up exactly when expected and stays for 5 days. Last month was no different, but for some reason Glo is showing my expected AF date this month on Day 27. Is there something I'm not taking into consideration that the app is?