Aparently the VideoLan Media Player isn't able to render the empty text chains ("") of a TextSample tag in a TTEXT subtitle muxed into a MP4 container.
This empty chains are used to clean the last shown subtitle at the ending time in this way:
<TextSample sampleTime="ending time" text="">
<\TextSample>
if they aren't rendered the subtitles stay in the screen until the next subtitle is shown or until the end of the video.
Actually i solve the problem putting a blank space into each empty chain (" ").