The title says it already:
Sometimes completed videos are not marked nor counted as completed.
This does not happen each time. Just sometimes.
I didn’t recognize some system of when it happens. But happen it does.
It’s annoying insofar as the progress bar also does not advance. And logging out and in again, the video is re-suggested as the next one to watch.
So you end up not really knowing what you did already watch.
In a course of 27 parts, this is not exactly practical.