- [X] I looked for a similar issue and couldn't find any.
- [X] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [X] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Hello. I have a problem about muxing WEBVTT subtitle to mp4box. On muxed mp4 video using mp4box with WEBVTT won't show the subtitle only in mpv or VLC.
In mkvtoolnix muxed mkv with webvtt, it shows correct subtitle.
See the two screenshots and shared files below:

WEBVTT texts:
```
WEBVTT

00:00.000 --> 00:02.000
Test
```

OS: Windows 11 Pro 21H2

mp4box version: 2.1-DEV-rev259-gfc4749f9-master

mp4box + mkvtoolnix muxed file: https://mega.nz/folder/nxVlgKYZ#HMLcZzKwtVDgIpSAAG6h3Q

mkvtoolnix muxed:
![mkvtoolnixmuxed](https://user-images.githubusercontent.com/88035011/185731203-1c4c1749-4d7e-42dc-a6d4-a533c998349f.png)

mp4box muxed:
![mp4boxmuxed](https://user-images.githubusercontent.com/88035011/185731221-67a3fef3-2172-4c7b-b0eb-6461f63bb817.png)

- Martin Eesmaa
Format "wvtt" of WEBVTT subtitle muxed of mp4box won't show subtitle.
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Format "wvtt" of WEBVTT subtitle muxed of mp4box won't show subtitle.
Issue date:

--- start body ---
- [X] I looked for a similar issue and couldn't find any.
- [X] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [X] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Hello. I have a problem about muxing WEBVTT subtitle to mp4box. On muxed mp4 video using mp4box with WEBVTT won't show the subtitle only in mpv or VLC.
In mkvtoolnix muxed mkv with webvtt, it shows correct subtitle.
See the two screenshots and shared files below:

WEBVTT texts:
```
WEBVTT

00:00.000 --> 00:02.000
Test
```

OS: Windows 11 Pro 21H2

mp4box version: 2.1-DEV-rev259-gfc4749f9-master

mp4box + mkvtoolnix muxed file: https://mega.nz/folder/nxVlgKYZ#HMLcZzKwtVDgIpSAAG6h3Q

mkvtoolnix muxed:
![mkvtoolnixmuxed](https://user-images.githubusercontent.com/88035011/185731203-1c4c1749-4d7e-42dc-a6d4-a533c998349f.png)

mp4box muxed:
![mp4boxmuxed](https://user-images.githubusercontent.com/88035011/185731221-67a3fef3-2172-4c7b-b0eb-6461f63bb817.png)

- Martin Eesmaa
--- end body ---
