Summary

The TwentyOne Contract uses the transfer method to send ETH in the endGame function to the winner:
TwentyOne L#170

Vulnerability Details

transfer() is not recommended to use as it has a set gas stipend. If the user is a contract that has a fallback function that consumes more than the gas stipend, the user will not receive the funds. (such as a Safe multisig).

Impact

The owner may not receive the payout.

Tools Used

manual code review/grep

Recommendations

Consider using the send or call functions to send the payout to the winner.