If this happens, please terminate the card right away from within the "Cards" section of the app. Then you can order a replacement card:
Go to Cards tab
Select the card that has been lost at the ATM
Tap “Manage”, “Report an issue”
Select "My card has been lost or damaged"
Tap “Report and order new card”
To understand why your card was swallowed, you will need to contact the ATM provider directly as we have no control over this type of incident.