As quick solution, you can simply open the file with mkvtoonix-gui, open the header editor and your file and change the 'force display' flag.
Haven't looked at the source code of MKV Cutter for quite some time, but I checked and MKV Cutter, does not set a flag that a subtitle file should be a default or forced track:
// SUBTITLE FILES
foreach(QString file, subtitleFiles)
{
options << "--no-video";
options << "--no-audio";
options << "--no-global-tags";
options << "--no-chapters";
options << "--no-track-tags";
options << "--subtitle-tracks";
options << "0";
lang = file;
index = lang.lastIndexOf("_track_");
if (index != -1) {
lang = lang.remove(0, index + 7);
lang = lang.remove(lang.lastIndexOf("."), lang.size()).trimmed();
index = lang.toInt();
lang = subtitles.at(index).language;
if (lang != QString()) {
options << "--language";
options << "0:" + lang;
}
}
options << "--compression";
options << "-1:none";
options << doubleBackSlash(file);
}
MKV Cutter doesn't even check whether a stream is flagged as forced or not.
Since I wrote MKV Cutter just as a proof of concept years ago, I don't really have any plan to change it in the near future.
So, the quickest option is to use the mkvtoolnix header editor.
Cu Selur
Ps.: Happy that MKV Cutter is still useful to some users out there.