The EvolvingProteus._reserveTokenSpecified function is called to calculate the changed amount of the LP token supply based on the proportional change of the utility of the pool. 
Firstly the initial utility and final utility are calculated as shown below:
Then, the proportional change of the utility is used to calculate the final LP token supply amount as shown below:
The issue here is the computed sf (final LP token supply amount) is not checked against the MIN_BALANCE value. Since _reserveTokenSpecified is called by both EvolvingProteus.depositGivenInputAmount and EvolvingProteus.withdrawGivenOutputAmount functions, the sf amount can decrease from the initial LP token supply amount of si during reserve token withdrawals.
This could prompt the sf < MIN_BALANCE condition to occur. If this condition occurs, the transaction should revert as it is done in the EvolvingProteus._getUtilityFinalLp function. But the transaction will not revert in the _reserveTokenSpecified function execution since there is no conditional check to verify sf >= MIN_BALANCE. 
The withdrawGivenOutputAmount transaction will execute successfully without reverting, even if the key invariant sf >= MIN_BALANCE is broken. Hence, this breaks the expected behaviour of the protocol.
https://github.com/code-423n4/2023-08-shell/blob/main/src/proteus/EvolvingProteus.sol#L586-L594
https://github.com/code-423n4/2023-08-shell/blob/main/src/proteus/EvolvingProteus.sol#L658
https://github.com/code-423n4/2023-08-shell/blob/main/src/proteus/EvolvingProteus.sol#L441-L448
VSCode
It is recommended to add the conditional check to verify sf >= MIN_BALANCE in the _reserveTokenSpecified function, after the sf value is calculated, as shown below. If the invariant for MIN_BALANCE is broken, then the transaction should revert.
