ExoPlayer PGSSUB implementation doesn't correctly render multiple on-screen captions

[silentTeee]

Describe the bug

Cross-post of jellyfin/jellyfin-android#1045, as this issue affects all Android apps that uitlize ExoPlayer and enable PGSSUB native rendering. Issue was introduced with #2690, and seems to be caused by google/ExoPlayer#7458

Logs

No response

FFmpeg logs

No response

Media info of the file

Mob Psycho 100 S01E09
Container: mkv
Size: 4530 MB

Subtitle
Title: Forced Only - English - PGSSUB
Language: eng
Codec: PGSSUB
Default: No
Forced: Yes
External: No

Audio
Title: Surround 5.1 - English - TRUEHD - Default
Language: eng
Codec: TRUEHD
Layout: 5.1
Channels: 6 ch
Bitrate: 2623 kbps
Sample rate: 48000 Hz
Bit depth: 24 bit
Default: Yes
Forced: No
External: No

Video
Title: 1080p H264 SDR
Codec: H264
AVC: Yes
Profile: High
Level: 41
Resolution: 1920x1080
Aspect ratio: 16:9
Interlaced: No
Framerate: 23.976025
Bitrate: 26170 kbps
Bit depth: 8 bit
Video range: SDR
Video range type: SDR
Pixel format: yuv420p
Ref frames: 1
NAL: 4

Audio
Title: Surround 5.1 - English - Dolby Digital
Language: eng
Codec: AC3
Layout: 5.1
Channels: 6 ch
Bitrate: 448 kbps
Sample rate: 48000 Hz
Default: No
Forced: No
External: No

Audio
Title: Stereo - Japanese - TRUEHD
Language: jpn
Codec: TRUEHD
Layout: stereo
Channels: 2 ch
Bitrate: 839 kbps
Sample rate: 48000 Hz
Bit depth: 24 bit
Default: No
Forced: No
External: No

Audio
Title: Stereo - Japanese - Dolby Digital
Language: jpn
Codec: AC3
Layout: stereo
Channels: 2 ch
Bitrate: 192 kbps
Sample rate: 48000 Hz
Default: No
Forced: No
External: No

Subtitle
Title: English - PGSSUB
Language: eng
Codec: PGSSUB
Default: No
Forced: No
External: No

Application version

0.15.7

Device information

HiSmartTV A4

Android version

Android 9

Jellyfin server version

10.8.10