What I wrote on https://bugzilla.opensuse.org/show_bug.cgi?id=1171770#c30 could be the kernel expected behavior. Instead of trying to set all interfaces to 1, it will be a no-op if the conf/all/forwarding was 1 already (not cycled). If we confirm that on Kubic the interfaces ip_forward are set to 0 as well, then we have to find out what is setting them to 0 and when.