That's not a standard macOS user account password.
Just checked on my Ventura system, and can change to new password with 8 characters or more, plus a mix of characters/numbers/special characters.
Something else is modifying/adding to what the default account login requires.
(Did you restart after uninstalling that portal app - whatever it was)
I still use a 5-character account login, even on Ventura, same password that I have used for this account for at least 20 years, and is my original password since early in Mac OS X days.
Maybe it is tied to whatever security level you are using with your AppleID? I choose to not use the upgraded security on that, so do not need to get an authentication code from another device. I use my login as a one-time login on other hardware frequrently, so still avoid that (to me) inconvenience of having to enter an authentication code. I know there is no going back to the old way, once I decide to "upgrade security", and, so far, Apple does not force the upgrade.
Maybe that is also a difference with your login?