The Istanbul hardfork increases the gas cost of the SLOAD operation and therefore breaks some existing smart contracts.
In file withdrawable.sol, contract uses transfer() to send eth from contract to EOA due which eth can get stuck.
The reason behind this is that, after the Istanbul hardfork, any smart contract that uses transfer() or send() is taking a hard dependency on gas costs by forwarding a fixed amount of gas (2300). This forwards 2300 gas, which may not be enough if the recipient is a contract and the cost of gas changes.
Recommend using call() to send eth.
fairside-core (FairSide) confirmed (separately in issue #67):
