If the second card dealt to a split ace is a 10-point card you do not receive the blackjack bonus for this hand. It does however win against an ordinary 21 made of more than two cards. If the dealer also has a blackjack the result for this hand is a push as usual. In many places the same rule (no blackjack bonus) is played if an ace is dealt as the second card to a 10-point card after splitting.
Source: https://www.pagat.com/banking/blackjack.html