Segmented subtitles not handled properly

[Xewdy444]

Describe the bug
Subtitle tracks listed in MPD files with a startNumber value greater than 1 are not downloaded properly. For example, this subtitle track should be downloaded as 9 segments and merged together accordingly:

<AdaptationSet id="51" lang="en-US" contentType="text" segmentAlignment="true">
    <Label>en-US</Label>
    <Role schemeIdUri="urn:mpeg:dash:role:2011" value="subtitle"/>
    <BaseURL>https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/</BaseURL>
    <Representation mimeType="text/vtt" id="t15" bandwidth="7">
        <SegmentTemplate timescale="1000" presentationTimeOffset="8132916" startNumber="9" media="t/0_23e44d/t15/$Number$.vtt">
            <SegmentTimeline>
                <S t="8132916" d="1814103"/>
            </SegmentTimeline>
        </SegmentTemplate>
    </Representation>
</AdaptationSet>

The segment URLs would be:

https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/1.vtt
https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/2.vtt
https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/3.vtt
https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/4.vtt
https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/5.vtt
https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/6.vtt
https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/7.vtt
https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/8.vtt
https://gcp.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/t/0_23e44d/t15/9.vtt

Devine instead only downloads segment 9.

MPD URL: https://akm.prd.media.h264.io/e58e0cad-2e65-463e-baaa-825862402a5a/1_7260bd_fallback.mpd

To Reproduce
Attempt to download an MPD subtitle track with a startNumber value greater than 1.

Expected behavior
The subtitle segments are all downloaded and merged accordingly.