Forced subtitles in my MKV files with forced flag set are not shown by default in MP 1.6 (same in MP 1.6 prerelease) with MP configured to "Subtitles will only display forced subtitles" (LAV filters).
The same configuration works well in MP 1.5 (mkv forced subtitles are shown by default)....
The same configuration works well in MP 1.5 (mkv forced subtitles are shown by default)....