If you have the key to a ciphertext, you can decrypt it, but that doesn't make the cipher insecure. It is the same principle for the seed of a CSPRNG. In fact,

If you have the key to a ciphertext, you can decrypt it, but that doesn't make the cipher insecure. It is the same principle for the seed of a CSPRNG. In fact, a common cipher construction (ChaCha20, for instance) relies on a random stream seeded by the key, generated by the sender to encrypt data, which the recipient re-generates from that same key to decrypt the data.