`ttmlparse` can't handled fragmented input

Here is a dash source, it contains subtitles.if you use playbin2, GST_TTML_AUTOPLUG=1 gst-launch-1.0 playbin uri=https://rolandjon.github.io/test-streams/sample-mpd-with-subtitles/sample-mpd-with-subtitles.mpd it can be played well, but using playbin3 is always stuck. GST_TTML_AUTOPLUG=1 gst-launch-1.0 playbin3 uri=https://rolandjon.github.io/test-streams/sample-mpd-with-subtitles/sample-mpd-with-subtitles.mpd

I have tried almost all versions from 1.14 stable to git-master latest versions, and they all seem to be stuck in the same way.BTW,vlc and ffplay also work well,it seems that only playbin3 cannot playback.

In addition, don’t forget to set GST_TTML_AUTOPLUG=1, the mpd contains ttml subtitles