When executing the extension/ezfind/bin/php/updatesearchindexsolr.php script as root, and using the "--allow-root-user|-r" option, this option is not passed to the forked processes.
The consequence is that all sub-processes fail to execute properly. These sub-processes are the ones effectively fetching the content and pushing it to Solr.
At l.490 in the aforementioned file, the "siteaccess" option is ported to sub-processes. It might be an idea to do the same with the --allow-root-user|-r option.