When doing an upgrade, debmonitor send the update status before the package being actually upgraded. I noticed that when upgrading Jenkins on contint1001.wikimedia.org. https://debmonitor.wikimedia.org/packages/jenkins already shown the package upgraded but the ugprade hasn't actually be done.
$ sudo apt upgrade ... Get:1 http://apt.wikimedia.org/wikimedia/ jessie-wikimedia/thirdparty jenkins all 2.176.2 [77.2 MB] ... Fetched 88.1 MB in 1s (57.9 MB/s) No directory, logging in with HOME=/ INFO:debmonitor:Got 14 updates from dpkg hook version 3 INFO:debmonitor:Successfully sent the dpkg_hook update to the DebMonitor server
At this point I guess the update has been send. Then:
(Reading database ... 61447 files and directories currently installed.) ... Unpacking jenkins (2.176.2) over (2.176.1) ...
There is thus a possibility for a package to fail to upgrade but be listed as having been upgraded.