Comment 2 for bug 2003061

I can confirm the issue. For the example above, the generated configuration will have only one route:

$ cat systemd/network/10-netplan-vlan100.network
[Match]
Name=vlan100

[Network]
LinkLocalAddressing=ipv6
Address=10.0.0.1/24
Address=10.0.0.2/24
ConfigureWithoutCarrier=yes

[Route]
Destination=0.0.0.0/0
Gateway=10.0.0.100
Table=1001

[RoutingPolicyRule]
From=10.0.0.1
Table=1001

[RoutingPolicyRule]
From=10.0.0.2
Table=1002

I'm investigating the issue and working on a fix.

Thanks for reporting the problem.