Header editor. "Default track" column values not always represent actual values of "Default track" flag
Among situations when the "Default track" flag is actually set to "Yes" for a track, the "Default track" value is displayed as "Yes" when the "Default track" flag does not exist for this track and this is a single track of that type (video / audio / subtitles).
Although, maybe a single video / audio track is treated as default by most of the players, but for subtitle tracks, it's definitely wrong.