The current Leap 15.3 kernel 5.3.18-59.19 has the out-of-order UDP packets issue, but older Leap 15.3 kernel 5.3.18-57.3 from http://download.opensuse.org/distribution/leap/15.3/repo/oss/ is ok. To test this I ran "zypper install --oldpackage kernel-default-5.3.18-57.3.x86_64" and updated by boot onfiguration in yast2.