Encrypting with AES-256 is widely claimed to be post-quantum secure.
But let's take a case where you use an initial key that has less than 256 bits of real entropy. In this example, let's work with a 128-bit key that has 128 bits of real entropy. We stretch it with something commonly used, like PBKDF2 with 120,000 iterations. We use the 256-bit result as the encryption key for AES-256 to encrypt some secret.
Is the the secret still secure against brute force attacks by a quantum computer, the kind can halve (or worse) the effective security of a symmetric cipher using Grover's algorithm? (Edited to add:) We assume the attacker knows the specifics of the above scheme, but not they length of the entropy or the initial key.