Yes, the Flint app WILL NOT let you proceed to the next screen, if the current month is entered as the expiration date on the card you are trying to process. This is for your own protection as a merchant as some cards have a specific day as an expiration, which you cannot enter.
Furthermore, card issuing banks are very aware of this issue and usually send out replacement cards to their clients as early as two months in advance of the real expiration month.
Your client should have received a new card in the mail already.
Fact is, the Flint app will not allow you to process a card that has the expiration month as the current month.