This is an odd one. We are Moodle 5 on Windows using OpenID Connect. On Tuesday night we got reports that users logging in were getting greeted with "Hi <wrong name>". They were actually logged in as the wrong user and could see grades, assignments, etc.
Two things happened. On Monday we had a hard failure in our data center and the Moodle app and SQL servers went down hard. When power was restored Moodle came up and worked normally. It was over 24 hours later that we got the reports about being logged in as the wrong user.
The other thing is that on Monday we updated the login page to suppress the local authentication username and passwords, giving users only the option of Microsoft 365. I can't imagine this is tied to anything but it is a change.
Clearing the cache fixed the problem. We rebooted the app server to be sure and all is working now.
Here's the question. How could this happen? What happened? We are baffled.