Ran into an issue where I was banging my head on the wall. Created a new user GPO, targeted a specific user, linked the GPO to the user OU and it would not show up when doing gpresult /r. It was not in the applied or filtered out groups, just missing.
I checked that the "read" and "apply group policy" permissions were checked for the user. No WMI filtering, the GPO was linked and enabled. I even tried linking it to the computer OU, enforcing the policy in both the user and computer OUs and zero change.
The fix is "Domain computers" needs to have read access to the GPO. I did not test targeting a specific computer, or computer group, but the computer needs permissions to read the GPO even though it is targeting a user.