In the iOS and Android app's video player, there's a button in the lower right of the screen that allows you to make the playback speed of the video faster or slower than the default 1x. I often use this option to speed up long, monotonous videos.
When watching a video using a faster playback speed on my iPhone (usually 1.3x), the video is not marked as complete and I do not get the correct full amount of energy points for watching the complete video.
When watching a video using a faster playback speed on my Android devices, the video is marked as complete and I get the correct full amount of energy points. (The Android app is not impacted by this bug)
The bug has existed on iOS for a long time (I don't think it was introduced recently), and it still exists as of the most recent version 4.5.0. I have experienced the bug on an iPhone 6s and an iPhone 7 running every version of iOS that I've tried. I now have a large number of videos (probably 100+) that I watched fully on my iPhone but aren't marked as complete because of this bug before figuring it out, which is extremely frustrating.