Having more than just alphanumeric characters widens the domain of the password hash function, and this directly increases the difficulty of brute-force cracking. But having a such a small maximum password length is... puzzling, to say the least. I would accept passwords of up to 1 KiB in length.
With rainbow tables, even 11-character simple passwords like 'password123' can be trivially cracked, and as the number of password leaks show, not everyone is great at managing secrets and credentials.
I recommend all my friends and family to use a password manager like Bitwarden, and if they can't do that for some reason, at least use a 3-word passphrase separated by a hyphen.
The amount of times people have complained to me that this doesn't work because of low max-chars on passwords is insane.
I bet the rationale would be "anything over 12 characters will be too hard to remember and people will just write down the password."
It's easier for me to remember really long passphrases than even short alphanumeric strings - small maximum password lengths set my teeth on edge. The passwords should be getting hashed anyway right?