Since you fixed JENKINS-58595, the rssAll feed no longer has the server name. Before this fix it was only showing the server HTTP address and no path in the href elements, now it's other way round. The HTTP-address is missing and only an (incorrect) relative link is shown. Not sure why it no longer shows full href.
I can confirm that the rootUrl being set or not does not have any incidence on the problem. You can find as attachment example before SEC-534 fix, one after SEC-534 and finally one after the correction of JENKINS-58595.