I got a question from a colleague who was asking how long it took before a user that was added to Azure AD is available in Azure DevOps. The answer is simple; instant.
So why was this person asking this question then? The reason is that there is a difference in behavior depending if the user is already added to the Azure DevOps organisation or not.
If the user is not part of the organisation yet, when you search for the user by firstname and lastname, nothing will be found.
But if you search for the user by his email address, you will find the user.
It is only after the user is added for the first time to an Azure DevOps organisation that you will be able to find a user by his name. Until then you should use the email address to search for the user in Azure AD.