Comment 21 for bug 1970139

Sorry for the incomplete search...

Apparently this is sort of a "known bug", due to the fact that when in "auto-login" the keyring is not unlocked then VNC server cannot access the stored password and creates a new random one.

https://askubuntu.com/questions/1403943/22-04-remote-desktop-sharing-authentication-password-changes-every-reboot

There is a semi-insecure fix proposed.

I'll try that until a better fix is proposed!