Before returning the duration value, the gst_mpd_client_get_next_fragment_duration function tries to validate it. The condition is:

    if (GST_CLOCK_TIME_IS_VALID (duration) || segments_count == 0
        || seg_idx < segments_count)
      return duration;
    return 0;

But it should be:
    if (GST_CLOCK_TIME_IS_VALID (duration) && (segments_count == 0
        || seg_idx < segments_count))
      return duration;
    return 0;

Even better, we could write the code similar to gst_mpd_client_get_next_fragment or gst_mpd_client_get_next_fragment_timestamp functions:

    if (!GST_CLOCK_TIME_IS_VALID (duration) || (segments_count > 0
            && stream->segment_index >= segments_count)) {
      return 0;
    }
    return duration;