Comment 1 for bug 2000596

Thanks for the thorough bug report. After some conversation in our IRC channel, we agree that your proposed idea of doing a check similar to:

if gateway not in ip_subnet:
    # add on-link to config

would be a good addition.

I noticed that you already submitted a PR at https://github.com/canonical/cloud-init/pull/1931 , so we'll track development there.