Cannot perform operations after a transaction has been committed or rolled back

Summary: I'm trying to implement a unique list (aka 'set') within a transaction (there's more to it than just that, otherwise I'd just use a lock, but this is minimum viable reproduction of a bug).

Repro:

1. Install/setup Couchbase Server locally
2. Create transactionTest bucket
3. Run attached console program

Expected: No exception thrown.

Actual: 

Couchbase.Transactions.Error.TransactionFailedException: 'Transaction failed.'

Inner exception
InvalidOperationException: Cannot perform operations after a transaction has been committed or rolled back.

Notes:

If I remove the if/rollback:

            if (myList.Contains("foo"))
            {
                await context.RollbackAsync();
                return;

                        }

then the code executes without exception, but then I'm left with ,["foo", "foo"] when I actually just wanted ["foo"]. 

More notes: I attached log output.