I have verified the fix on Jammy using systemd 249.11-0ubuntu3.10 from jammy-proposed:

root@clean-jammy-amd64:/home/nr# apt-cache policy systemd
systemd:
  Installed: 249.11-0ubuntu3.10
  Candidate: 249.11-0ubuntu3.10
  Version table:
 *** 249.11-0ubuntu3.10 500
        500 http://archive.ubuntu.com/ubuntu jammy-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
     249.11-0ubuntu3.9 500
        500 http://archive.ubuntu.com/ubuntu jammy-updates/main amd64 Packages
     249.11-0ubuntu3.7 500
        500 http://security.ubuntu.com/ubuntu jammy-security/main amd64 Packages
     249.11-0ubuntu3 500
        500 http://archive.ubuntu.com/ubuntu jammy/main amd64 Packages
root@clean-jammy-amd64:/home/nr# cat > /etc/systemd/network/25-dhcp-server-veth-peer.network << EOF
[Match]
Name=veth-peer

[Network]
IPv6AcceptRA=no
Address=2600::1/0
Address=192.168.5.1/24
EOF
root@clean-jammy-amd64:/home/nr# cat > /etc/systemd/network/25-test.network << EOF
[Match]
Name=veth99

[Network]
DHCP=ipv4
IPv6AcceptRA=false

[DHCPv4]
UseRoutes=yes
EOF
root@clean-jammy-amd64:/home/nr# cat > /etc/systemd/network/25-veth.netdev << EOF
[NetDev]
Name=veth99
Kind=veth
MACAddress=12:34:56:78:9a:bc

[Peer]
Name=veth-peer
MACAddress=12:34:56:78:9a:bd
EOF
root@clean-jammy-amd64:/home/nr# mkdir -p /etc/systemd/system/systemd-networkd.service.d/
root@clean-jammy-amd64:/home/nr# cat > /etc/systemd/system/systemd-networkd.service.d/debug.conf << EOF
[Service]
Environment=SYSTEMD_LOG_LEVEL=debug
EOF
root@clean-jammy-amd64:/home/nr# systemctl daemon-reload
root@clean-jammy-amd64:/home/nr# systemctl restart systemd-networkd
root@clean-jammy-amd64:/home/nr# mkdir -p /run/networkd-ci
root@clean-jammy-amd64:/home/nr# dnsmasq --log-facility=/run/networkd-ci/test-dnsmasq.log --log-queries=extra --log-dhcp --pid-file=/run/networkd-ci/test-dnsmasq.pid --conf-file=/dev/null --bind-interfaces --interface=veth-peer --dhcp-leasefile=/run/networkd-ci/test-dnsmasq.lease --enable-ra --dhcp-range=2600::10,2600::20,2m --dhcp-range=192.168.5.10,192.168.5.200,2m --dhcp-option=option:mtu,1492 --dhcp-option=option:router,192.168.5.1 --port=0 --no-resolv --dhcp-option=option:classless-static-route,0.0.0.0/0,192.168.5.4,8.0.0.0/8,192.168.5.5,192.168.5.64/26,192.168.5.5 &
[1] 1418
root@clean-jammy-amd64:/home/nr# 
[1]+  Done                    dnsmasq --log-facility=/run/networkd-ci/test-dnsmasq.log --log-queries=extra --log-dhcp --pid-file=/run/networkd-ci/test-dnsmasq.pid --conf-file=/dev/null --bind-interfaces --interface=veth-peer --dhcp-leasefile=/run/networkd-ci/test-dnsmasq.lease --enable-ra --dhcp-range=2600::10,2600::20,2m --dhcp-range=192.168.5.10,192.168.5.200,2m --dhcp-option=option:mtu,1492 --dhcp-option=option:router,192.168.5.1 --port=0 --no-resolv --dhcp-option=option:classless-static-route,0.0.0.0/0,192.168.5.4,8.0.0.0/8,192.168.5.5,192.168.5.64/26,192.168.5.5
root@clean-jammy-amd64:/home/nr# 
root@clean-jammy-amd64:/home/nr# 
root@clean-jammy-amd64:/home/nr# 
root@clean-jammy-amd64:/home/nr# systemctl restart systemd-networkd
root@clean-jammy-amd64:/home/nr# ip r show dev veth99 192.168.5.64/26
192.168.5.64/26 via 192.168.5.5 proto dhcp src 192.168.5.181 metric 1024 
root@clean-jammy-amd64:/home/nr# journalctl -u systemd-networkd --grep="veth99.*assigned network"
-- No entries --