The overloaded method org.dspace.statistics.SolrLogger#getCommonSolrDoc(org.dspace.content.DSpaceObject, java.lang.String, java.lang.String, java.lang.String, org.dspace.eperson.EPerson) as opposed to org.dspace.statistics.SolrLogger#getCommonSolrDoc(org.dspace.content.DSpaceObject, javax.servlet.http.HttpServletRequest, org.dspace.eperson.EPerson) only logs the ip is useProxies is enabled and something is filled in in X-Forwarded-For. This is because the entire block logging data derived from the IP is wrapped in if (isUseProxies() && xforwardedfor != null) while it should only be the determination of the ip.
Luckily this code doesn't usually get triggered as usually the UsageEvent will contain a reference to the ServletRequest but someone will eventually use this code.