Bad password fields like this are everywhere. Some of my favorites:
1. The ones that don't tell you what the restrictions are at all, just that your password is "wrong". 2. Tells you that a special character is required but doesn't tell you which ones are valid and which ones aren't. 3. Password requirements as stated do not match how the password field validates the password. (My favorite which I see a lot: Must be 20 characters or less... only accepts up to 19 characters.) 4. Allows only certain length of password on account creation, truncating the password your manager enters, but the login form password field is unlimited. 5. Silently truncates your password without telling you, even if you enter a longer password. 6. The ones that are either intentionally or unintentionally written such that somehow your password manager can't even find the form fields. Most especially the password field. 7. The ones that won't let you type or paste into them but instead give you an on-screen keyboard you have to key in your password with mouse clicks on a virtual keyboard for "security".
I could go on for days, but there's a lot of really badly written websites out there. At least in this case it was abundantly clear immediately that it didn't accept the bracket. I'd daresay call that login form good compared to some of the crap I've seen.