Converting MKV to MKV doesn't show Dutch subtitles

[WIZZs]

Hi,
My actual version is 10.0.6.2 (downloaded totday).
I'm trying to convert a MKV to MKV (just to decrease it's filesize).
Even though the MKV does contain Dutch subtitles, they aren't shown in the list (only English is shown, which is also availeble in the MKV).