@FredS,
I think that if an update is interrupted or aborted; or the network isn't available during the set time for a reboot an update has the possibility of being skipped. I will need to look for more information on the rules that are applied in these situations for verification. Due to the risks that come with upgrading a router's firmware, such as the possiblity of bricking the device, I would assume that the rules and protections in place are more likely to halt an iffy upgrade, rather than let it happen and possibly fail.
We always recommend checking for updates both through the UI and by comparing versions with our support pages, as it is possible that connections with our Cloud through your ISP may be impacted, causing updates not to appear.
Also, as an FYI, the auto-update feature is still relatively new, with most devices only receiving the update last year- so I imagine that the reliability and rules of the feature will improved across updates.