I am upgrading a range of hosts from 13.2 to 42.1 and on one host a surprising
issue emerged right after the restart after the upgrade: The default gateway is
not set anymore.

If I use yast to generate the route configuration (in
/etc/sysconfig/network/routes)
default 10.1.2.1 - -

the default gateway is not set if explicitly select the interface yast generate
(in /etc/sysconfig/network/ifroute-eth0)
default 10.1.2.1 - eth0

which works.

Please note, that the configuration without the explicit interface works on
other hosts with the exact same configuration inside Yast. The only difference
is their IPs.

This happens with Wicked and a statically assigned IP.