Well - I can tell you how to fix this one particular way (its the only way I know how) but it will open up another can of worms for you.
The issue is that in Ubuntu 22.04, all the username and passwords for your system are, by default, encrypted with your username's password. That is being stored in what is called the keyring. What is happening is that the VNC server is being loaded without the keyring being unlocked and therefore can't access the stored VNC password (as its encrypted) so it auto generates a new one each time you boot up.
So, if you auto boot up with your user login, during the auto boot up, Ubuntu 22.04 doesn't automatically unlock the keyring. So to fix this, go to Utilities, Password and Keys, right click on Default Keyring, select change password. It will ask for your username password. Enter that. Then for the new password, do not enter one. Leave it blank. It will warn you that all passwords stored on your keyring will become and remain unencrypted. If you can wear that risk, accept that.
Then go back to remote desktop, ensure the password you want is set, and each time you reboot, that password will then remain the same.
Hope that all makes sense :)