I didn't fully understand what the bug was, but I found on a German news page[1] with a slightly more detailed description:
Aufgezeichnet wurde zwar die Gesamtsehdauer aller User, geteilt wurde dieser Wert allerdings nur durch diejenigen User, die das Video länger als drei Sekunden ansahen
So the average was not calculated correctly: They accumulated the duration of all video views (including of those shorter than 3 seconds), but divided it by the number of 'legitmate' views - i.e. only those longer than 3 seconds. So you get a pretty big offset if you have many views under 3 seconds (which they probably do, thanks to autoplay).
Aufgezeichnet wurde zwar die Gesamtsehdauer aller User, geteilt wurde dieser Wert allerdings nur durch diejenigen User, die das Video länger als drei Sekunden ansahen
So the average was not calculated correctly: They accumulated the duration of all video views (including of those shorter than 3 seconds), but divided it by the number of 'legitmate' views - i.e. only those longer than 3 seconds. So you get a pretty big offset if you have many views under 3 seconds (which they probably do, thanks to autoplay).
[1] https://www.heise.de/newsticker/meldung/Facebook-Unklare-Vid...