I requested a password reset, didn't click the link, then logged in with the current password, and in the UI there was a yello notification with:

WARNING: plugins/Login/PasswordResetter.php(278): Notice - Undefined index: password - Matomo 3.6.1-b1 - Please report this message in the Matomo forums: https://forum.matomo.org (please do a search first as it might have been reported already)