Hello!
This is not a bug with Notes. Whenever you lock a note with passwords you will need that password to re open them. When you reset the password you're changing the password for all future notes, not the previous ones. The notes page you're referring to is locked with one of your previous passwords.
When you reset your password, your old locked notes are not deleted. Once you remember the old password you can open those notes and choose to update them to the new password.
Please read this support page: How to lock or unlock notes on your iPhone, iPad, and iPod touch - Apple Support.
"If you forgot the password, Apple can't help you regain access to your locked notes. You can create a new password, but this won’t give you access to your old notes. A new password will let you password protect any notes that you create going forward with the new password.
If you have multiple passwords, you won't be able to tell which password you should use when you look at your Notes list. When you open a note locked by your old password but you enter your current password, you'll see an alert that you entered the wrong password with a hint for your old password. If you enter the correct old password, you get the option to update that note's password to your current one." - From the Support page above.
iOS 16, coming later this year, will allow you to lock notes with Face ID and your iPhone's passcode instead of a newly created one. However, you will still need to unlock your password protected notes before updating to this new password structure.
I hope this answers your question!
