It usually takes a couple of days, if not a one or two weeks, for your phone to download and update your apps in the background and learn your personal power usage, so if you 'just got' your replacement, just be patient and see if the issue doesn't resolve itself in due time. In the meantime, you can check your Google Assistant settings and ensure that it has the most current version.
Go into your settings and go to hey Google & voice match and turn on the settings, but the ultra 23 is set up for Bixby and turned off Google automatically because they want us to use the android siri lol
Goto settings than Google account option check everything is good there I would think there is option there or another setting option that has the assistance voice in it and check the playstore for any update if I recall when you do the initial setup it's one of the options they ask during setup i.e Google voice and or assistant
