If you get to the end of the video when it plays in the background, and then you wait a few minutes after it ends and come back to the app - when you seek backwards, the video will be black.
You need to close the video and open it again for it to work. The audio usually still works.
Power saving mode was on - not sure if it matters.
Edit:
I think it doesn't matter if its in the end or not - if you leave the app and come back, it will happen.
If you get to the end of the video when it plays in the background, and then you wait a few minutes after it ends and come back to the app - when you seek backwards, the video will be black.
You need to close the video and open it again for it to work. The audio usually still works.
Power saving mode was on - not sure if it matters.
Edit:
I think it doesn't matter if its in the end or not - if you leave the app and come back, it will happen.