Compiler fails to do assignment-to-final-fields checks on fields made final by @Immutable (was: @Immutable does not make fields immutable)

@Immutable
class Account {
    BigDecimal balance
    String customer
    
    Account deposit(amount) {
        balance = balance + amount
        this
    }
    
    String toString() {
        "Account[balance: $balance, customer: $customer]"   
    }
}

def acc = new Account(0.0, "Test")
acc.deposit(3.1) //should raise exception, shouldn't it?
assert 3.1 == acc.balance