That's not a requirement. Pigpen is a substitution cipher.
You will find that the pigpen cipher has a 1:1 mapping between its input alphabet and its output alphabet, and that a 1:1 mapping is a necessity for full invertibility.
You will find that the pigpen cipher has a 1:1 mapping between its input alphabet and its output alphabet, and that a 1:1 mapping is a necessity for full invertibility.