MKV files use a "Cues" element to act as a map for the video data. Without it, your player doesn't know where a specific second of video is located on your hard drive.
: A versatile player that also includes basic tools for merging and editing metadata. mkv index
: Some players, like mpv , may explicitly report a "discarding broken index" warning. MKV files use a "Cues" element to act