When I use MediaCoder to render *** subtitle into a mp4 file (using x264 encoder), the first 8 secs of video output is darkened like phantom screens (i.e. the faces and backgrounds are all blackened). I have no problem if I use external ffmpeg command e.g.
ffmpeg -i taipeisky.mp4 -vf "***=taipeisky.***" -y taipeisky_1.mp4