Display time of subtitles are changed to almost zero after using MKVToolNix with DVD's

I would like to report the following issue: When I want to add a cover to a DVD in MKV format with MKVToolNix the display time of subtitles are so short that it is impossible to read the text. I seems that there are flashed using Media Player Classic. This issue only happens with DVD's.

Using the VLC Player instead of Media Player Classic this issue will not appear. Using the original file - without adding a cover the file - the Media Player Classic works fine.

Hope you can fix it.