The bug
When you try to set a password you have used before it shows you "Not strong enough! Must be at least 8 characters. Mix numbers, letters and symbols." even if the password matches these criteria. This can be quite confusing. Showing a more appropriate message might be better (unless it reveals too much about how the user chooses their passwords).
The system might be slightly bugged IF this is not supposed to happen in the first place. I agree with the better message, if it was supposed to happen, yet appears wrong.
I can see the message being "Try a different password, this one might be a easy to figure out".
Then again, it might have already been fixed, it's 13 Nov 2018 that I'm posting this.