I’ve run into a lot of issues stemming from people creating personal Microsoft with the same email as their O365 email(why Microsoft? Fucking why do we even allow this?)
So they’ll mess around and log into their personal on one app and their work email on another, and then everything goes to shit because I guess Microsoft doesn’t quite grasp that having two accounts to very similar systems that have the exact same username is asinine.
Anyway, make sure you remove every account from the system, including removing the device from any azure domain, rejoin, and hand hold users through signing it.
We had this a lot during Covid where people just registered free Teams accounts with their work e-mail adresses. These environments are super small so I will check it out to be sure!