"Run as" user button does nothing

I’m trying to run as users so that I can add bulk listings to accounts on their behalf as part of our onboarding process. In the past I was able to have this work, but it doesn’t seem to work now. I’m on the index page, go to live data, users, then run as the user. It then runs the page, with me logged in as who I was before or not logged in if I wasn’t.