This is a classic Feistel network round: L_new = L_old ^ F(key, R_old). It's a deliberate design.
The security of the Labyrinth relies on the composition of many rounds, not any single one. The unchanged right half from a Hash round is fully encrypted if the next round is a Stream round. Since the Labyrinth is a deep, aperiodic mix of both round types, the entire block is guaranteed to be diffused and encrypted.
The security of the Labyrinth relies on the composition of many rounds, not any single one. The unchanged right half from a Hash round is fully encrypted if the next round is a Stream round. Since the Labyrinth is a deep, aperiodic mix of both round types, the entire block is guaranteed to be diffused and encrypted.