A video added as a layer will have slashed dark lines when the video's resolution is too high.
This problem can be fixed by lowering the resolution of the video you are trying to add as a layer. For how to lower the resolution of a video using KineMaster, please refer to How can I make a lower resolution copy of a video?
If the problem persists, please contact us via Email Support below, with the followings attached:
• a screenshot showing the timeline
• a screenshot showing the video layer information, which you can get by tapping on the video layer and then tapping the (i) Information button.