For laptop users in particular, the first run of apt-daily.service each day is likely to occur on boot or after resume from suspend. Such a run will fail to update package lists due to a DNS failure because of #834414. The problem is made worse, however, because of apt's current exit code behavior. After a failure to fetch, apt still exits with 0, and so the apt.systemd.daily script considers it successful and touches the update-stamp. Accordingly, all future runs of apt-daily.service that day will check the update-stamp, see that it is recent, and abort running. So, even if apt-daily.service has a chance to run later in the day when the laptop is awake and network is up, it still will fail to update package lists.
It seems that either apt's exit code behavior needs to change according to this bug report, or if this is intended behavior then apt.systemd.daily needs to check for DNS failure and handle it differently, in which case perhaps a new bug report needs to be filed. Can someone advise?
When #834414 is fixed, this will not be as common of an issue, but it still remains a separate bug as it is imaginable that the first run of apt-daily.service in a given day could happen while DNS is down for any number of reasons.