Sometime, users can't be found in our people pickers. This could have happened while adding a user: -in a team -in a template -in a people tag -even as a Powell Teams administrator.
Powell Teams use the Graph API to get the users of your tenant and display them into our people pickers. All users have a "UserType" attribute that should be filled to "member" or "guest" to find them within Graph API.
This new attribute has been introduced by Microsoft in the last few years. Users that had been created before this update still have their "UserType" attribute defined as "blank".