I am noticing the same problem. I just started AWS Big Data Speciality course. After I comleted watching the IoT Registry video I clicked on the "Mark as Complete and Start Next Video" button. The next video that displayed was "Iot Rules" which is incorrect. It skipped couple of videos following the "IoT Registry" video. I am having to go back to the course content every time after completing a video.
I don't think it's specific to a certain video/course. I think it goes deeper than that because I've seen this behavior on three different courses so far., along with "Complete Section" resulting in going to the video I just watched.