If the Display Name is manually specified in the invite, the External Azure AD should not override and rewrite it.
When I set up a guest user in Azure AD, and they accept the invitation, the external Azure AD rewrites their display name in a scheme that doesn't match either organization's naming convention. As the users accept the invites, the admin has to go back and rewrite the guest's display name. Doing this for massive numbers of users becomes cumbersome. This property should be able to be locked on the inviting AD.