Hello,
This issue appears to be a known issue with Apache and not specific to GCE instances. This issue has been addressed in Serverfault. “In bugzilla, mod_remoteip fills in %a while it removes from %{X-Forwarded-For}i. So in a simple case with one trusted proxy, %a will hold the value used to see in X-Forwarded-For because of mod_remoteip”