VTT position data is ignored

mpv information:

[anthony@archlinux tmp]$ mpv --version
mpv v0.40.0-dirty Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
 built on May 21 2025 20:53:16
libplacebo version: v7.351.0
FFmpeg version: n7.1.1
FFmpeg library versions:
   libavcodec      61.19.101
   libavdevice     61.3.100
   libavfilter     10.4.100
   libavformat     61.7.100
   libavutil       59.39.100
   libswresample   5.3.100
   libswscale      8.3.100

Other information/system information:

• Linux version: Arch (installed the easy way on this system but I have done it the real way before)
• Kernel Version: Linux archlinux 6.15.2-arch1-1 #1 SMP PREEMPT_DYNAMIC Tue, 10 Jun 2025 21:32:33 +0000 x86_64 GNU/Linux
• GPU Model: 05:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Picasso/Raven 2 [Radeon Vega Series / Radeon Vega Mobile Series] [1002:15d8] (rev c2)
• Mesa/GPU Driver Version: OpenGL version string: 4.6 (Compatibility Profile) Mesa 25.1.4-arch1.1
• Window Manager and Version: sway-1:1.11-1
• Source of mpv: Not AUR, repos
• Latest known working version: This issue probably has been around for ages.
• Issue started after the following happened:

Reproduction Steps:

(with yt-dlp):

yt-dlp https://www.youtube.com/watch?v=SsCfraMIWaA --write-subs

mpv "101 Dalmatians - Cruella De Vil's Outburst [SsCfraMIWaA].webm" --sub-auto=fuzzy --no-config

Expected Behaviour:

The subtitles should go under the speaker to idetify speakers. Though this style is fading out of style. (source)

Actual Behaviour:

The captions are just in the middle bottom and don't move at all.

Alternative behaviour of the wanted feature

The alternative is just not supporting it, and that is bad.
No response

Sample Files

https://www.youtube.com/watch?v=SsCfraMIWaA

log

output.txt

[anthony@archlinux ~]$ neofetch 
                   -`                    anthony@archlinux 
                  .o+`                   ----------------- 
                 `ooo/                   OS: Arch Linux x86_64 
                `+oooo:                  Host: 20NEA00MAU ThinkPad E495 
               `+oooooo:                 Kernel: 6.15.1-arch1-2 
               -+oooooo+:                Uptime: 6 hours 
             `/:-:++oooo+:               Packages: 1907 (pacman) 
            `/++++/+++++++:              Shell: bash 5.2.37 
           `/++++++++++++++:             Resolution: 1366x768 
          `/+++ooooooooooooo/`           WM: sway 
         ./ooosssso++osssssso+`          Theme: Yaru-Aubergine-dark [GTK2/3] 
        .oossssso-````/ossssss+`         Icons: breeze-dark [GTK2/3] 
       -osssssso.      :ssssssso.        Terminal: kitty 
      :osssssss/        osssso+++.       CPU: AMD Ryzen 5 3500U with Radeon Vega Mobile Gfx (8) @ 2.100GHz 
     /ossssssss/        +ssssooo/-       GPU: AMD ATI Radeon Vega Series / Radeon Vega Mobile Series 
   `/ossssso+/:-        -:/+osssso+-     Memory: 7031MiB / 13741MiB 
  `+sso+:-`                 `.-/+oso:
 `++:.                           `-/+/                           
 .`                                 `/                           


[anthony@archlinux ~]$ mpv --version
mpv v0.40.0-dirty Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
 built on May 21 2025 20:53:16
libplacebo version: v7.351.0
FFmpeg version: n7.1.1
FFmpeg library versions:
   libavcodec      61.19.101
   libavdevice     61.3.100
   libavfilter     10.4.100
   libavformat     61.7.100
   libavutil       59.39.100
   libswresample   5.3.100
   libswscale      8.3.100
[anthony@archlinux ~]$ [anthony@archlinux ~]$ 

####I carefully read all instruction and confirm that I did the following:

• I tested with the latest mpv version to validate that the issue is not already fixed.
• I provided all required information including system and mpv version.
• I produced the log file with the exact same set of files, parameters, and conditions used in "Reproduction Steps", with the addition of --log-file=output.txt.
• I produced the log file while the behaviors described in "Actual Behavior" were actively observed.
• I attached the full, untruncated log file.
• I attached the backtrace in the case of a crash.

Reproduce the issue with the provided yt-dlp and mpv commands, then inspect the attached output.txt log and the VTT subtitle-positioning path. Done means VTT position data is honored so captions move under the speaker, or the supported behavior is clearly established with appropriate coverage.