This is with the "AD Group to Profile: Advanced Settings" > "When to update" set to "When creating and updating a user" and the default profile ("If value is null apply default value") set to "Identity User".
I created two users in AD - icUser04 & icUser05 - neither of which were initially added to an AD profile group. After I started up IC, I mapped ProfileGroup1 to "Chatter Moderator User" and ProfileGroup3 to the lower priority "Inside Sales". After a full recon, both users were created in Salesforce and assigned to the default "Identity User" profile, as expected.
I then updated the two users by adding icUser04 to AD ProfileGroup1 and adding icUser05 to AD ProfileGroup3. After a full recon, I expected icUser04 to be reassigned to the Salesforce "Chatter Moderator User" profile and icUser05 to be reassigned to the Salesforce "Inside Sales" profile, but both were still assigned to the default "Identity User" profile.