> Can you try how the behavior is on Focal 20.04?

It works as expected on Focal as well; it seems Jammy really is the
outlier here xD

> Also can you please check the journalctl log for networkd complaining
about unsupported settings?

Here's the output of `sudo journalctl -u systemd-networkd.service` from a Jammy 
instance I launched and then added the overrides you suggested :) 
https://pastebin.ubuntu.com/p/vQqVgZYx4x
I can't see any complaints about unsupported settings, etc. though? 

> And provide the full "netplan status" / "ip addr" output, to see if
there's any IPv6 connection allowing this lookup? (As the settings
provided above mostly block it for IPv4.)

So GCP defaults to IPv4 only unless you specifically use a non-default 
DUAL_STACK or IPV6_ONLY network interface card. I've shared the logs here for 
prosperity but it's all IPv4. Happy to try in a DUAL_STACK/IPV6_ONLY set up 
though if you think it'd give helpful data :)
Here's the `netplan status` and `ip addr` outputs on the same VM as above 
though: https://pastebin.ubuntu.com/p/BQTTQQ3hMg/ 

> Another thing you could try is adding an override for
[DHCPv4].UseDomains=false (and potentially [DHCPv6].UseDomains=false &
[IPv6AcceptRA].UseDomains=false), to see if having it specified for the
explicit section makes it work

Thanks for the suggestion! I tried those configs as well but unfortunately they 
didn't work. But FWIW @virtustom may have figured out the exact commit needed 
to get this working in `systemd` on Jammy xD 
https://github.com/systemd/systemd/commit/0ce86f5e
Here's his rationale: "Before google-guest-agent makes its first contact to MDS 
systemd-networkd attempts to add static routes to DNS/NTP addresses received 
from DHCP leases of all interfaces. Note that only the DHCP lease of the 
primary interface contains a gateway address. The commit I linked above causes 
systemd-networkd to not add routes to DNS/NTP addresses on different networks 
without gateway. So on noble these routes are not added..."

I do think you're right though and it's a systemd snafu, specifically
that that commit isn't present in Jammy.

Thanks again!
Chlo

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https://bugs.launchpad.net/bugs/2131966

Title:
  Netplan on Jammy doesn't prevent metadata/DNS routing on a secondary
  NIC despite any DHCP overrides given

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