unaccept on community wiki still yields -15 reputation to the original poster, for instance "-15 11 hours ago unaccept".
However it actually does not change reputation but shows up in the reputation log. I am not perfectly sure, however I believe the post was made community wiki after it was accepted, in that regard the -15 rep. should have been factored into the rep. score. Now it just shows as -15 in the log only.
Even it's a minor bug I guess the entire reputation system doesn't not use the history logs but only the present state. If so, it should be resolved as a known issue.