If there isn't other reasons against it, I will suggest to rename the current preset av1.webm to av1_aom.webm
This way it inform the users which encoder it is, and also follows the related naming convention av1_svt, av1_vaapi, and possibly additional ffmpeg supported av1_encoders av1-rav1e and avif (image), and additional av1_hwaccels methods (qsv, amf, nvenc) later.
And possibly upgrade to svt-av1 v.2.1.

Transcoding regarding duration was also mentioned above.
My input file here was a cfhd transcoded with ffmpeg from a Prores 422 hq source video file (that is not a arbitrary cut).

Alt. 1)
Therefore I went back to my source Prores 422 (HQ) hd01.mov which especially default includes 16 channels LPCM audio, even though only 2 ch stereo is used.

ffprobe -hide_banner hd01.mov

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'hd01.mov':
Metadata:
    creation_time   : 2016-02-23T23:49:21.000000Z
Duration: 00:01:11.28, start: 0.000000, bitrate: 200496 kb/s
Stream #0:0[0x1](eng): Video: prores (HQ) (apch / 0x68637061), yuv422p10le(bt709, top coded first (swapped)), 1920x1080, 182130 kb/s, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 2500 tbn (default)
      Metadata:
        creation_time   : 2016-02-23T23:49:21.000000Z
        handler_name    : Apple Video Media Handler
        vendor_id       : appl
        encoder         : Apple ProRes 422 (HQ)
Stream #0:1[0x2](eng): Audio: pcm_s24le (lpcm / 0x6D63706C), 48000 Hz, 16 channels, s32 (24 bit), 18432 kb/s (default)
      Metadata:
        creation_time   : 2016-02-23T23:49:21.000000Z
        handler_name    : Apple Sound Media Handler
        vendor_id       : [0][0][0][0]

Loaded this Prores 422 hq source file into CinGG

1) Video only rendering - no error messages

Rendering to file /Videoklipp/Cineform/av1_aom_yuv422p10le.webm
File format; FFMPEG webm
Video only wrench: av1.webm | yuv422p10le
cin
Cinelerra Infinity - built: Jul 29 2024 01:18:06
....
Render::render_single: Session finished.
** rendered 1781 frames in 187.108 secs, 9.519 fps

FFMPEG::open_decoder: some stream times estimated: /run/media/terje/Videoklipp/Cineform/av1_aom_yuv422p10le.webm
FFMPEG::open_decoder: some stream times estimated: /run/media/terje/Videoklipp/Cineform/av1_aom_yuv422p10le.webm
FFMPEG::open_decoder: some stream times estimated: /run/media/terje/Videoklipp/Cineform/av1_aom_yuv422p10le.webm
Total excess of backups: -50
Session time: 0:05:08
Cpu time: user: 0:20:00.969 sys: 0:00:08.355

2) Audio only rendering - not possible in CinGG

Cinelerra Infinity - built: Jul 29 2024 01:18:06
-
16 channels lpcm audio (actually only #1 and #2 stereo used)

Rendering to file /Videoklipp/Cineform/opus.webm
File format; FFMPEG webm
Audio only wrench: opus.webm

[libopus @ 0x7f637c06a6c0] Opus mapping family undefined for 16 channels.
FFMPEG::open_encoder err: Invalid argument
int FFMPEG::open_encoder(const char*, const char*):
open failed libopus:/run/media/terje/Videoklipp/Cineform/opus.webm
Render::render_single: Session finished.
Total excess of backups: -50
Session time: 0:02:21
Cpu time: user: 0:00:13.680 sys: 0:00:02.102

-----------------

Alt. 2)
Transcoded alt. 1) source input file to Prores 422 hq with 2 ch stereo audio:

ffmpeg -hide_banner -i hd01.mov -map_metadata 0 -c:v copy -c:a pcm_s24le -ac 2 hd02.mov

ffprobe -hide_banner hd02.mov

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'hd02.mov':
Metadata:
    major_brand     : qt
    minor_version   : 512
    compatible_brands: qt
    creation_time   : 2016-02-23T23:49:21.000000Z
    encoder         : Lavf61.1.100
Duration: 00:01:11.28, start: 0.000000, bitrate: 184335 kb/s
Stream #0:0[0x1](eng): Video: prores (HQ) (apch / 0x68637061), yuv422p10le(bt709, top coded first (swapped)), 1920x1080, 182130 kb/s, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 10k tbn (default)
      Metadata:
        creation_time   : 2016-02-23T23:49:21.000000Z
        handler_name    : Apple Video Media Handler
        vendor_id       : FFMP
        encoder         : Apple ProRes 422 (HQ)
Stream #0:1[0x2](eng): Audio: pcm_s24le (in24 / 0x34326E69), 48000 Hz, stereo, s32 (24 bit), 2304 kb/s (default)
      Metadata:
        creation_time   : 2016-02-23T23:49:21.000000Z
        handler_name    : Apple Sound Media Handler
        vendor_id       : [0][0][0][0]

-----------------

Alt. 3)
Transcoded th source file Prores 422 (HQ) with LPCM audio to CFHD quality 7 (high+), AAC audio (default) to stereo (-ac 2) and Metadata (some)

ffmpeg -hide_banner -i hd01.mov -map_metadata 0 -c:v cfhd -quality 7 -ac 2 cfhd01.mov

ffprobe -hide_banner cfhd01.mov

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'cfhd01.mov':
Metadata:
    major_brand     : qt
    minor_version   : 512
    compatible_brands: qt
    creation_time   : 2016-02-23T23:49:21.000000Z
    encoder         : Lavf61.1.100
Duration: 00:01:11.28, start: 0.000000, bitrate: 181871 kb/s
Stream #0:0[0x1](eng): Video: cfhd (CFHD / 0x44484643), yuv422p10le(bt709, top coded first (swapped)), 1920x1080, 181846 kb/s, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 12800 tbn (default)
      Metadata:
        creation_time   : 2016-02-23T23:49:21.000000Z
        handler_name    : Apple Video Media Handler
        vendor_id       : FFMP
        encoder         : Lavc61.3.100 cfhd
Stream #0:1[0x2](eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 122 kb/s (default)
      Metadata:
        creation_time   : 2016-02-23T23:49:21.000000Z
        handler_name    : Apple Sound Media Handler
        vendor_id       : [0][0][0][0]

By loading Alt. 2) Prores LPCM or Alt. 3) CFHD AAC with 2 ch stereo audio into CinGG

1) Video only av1_aom.webm rendering caused the previous FFseek I/O errors

2) Audio only hd02.mov stereo lpcm -> opus.webm worked ok

cin
Cinelerra Infinity - built: Jul 29 2024 01:18:06
........
Render::render_single: Session finished.
** rendered 1781 frames in 6.387 secs, 278.848 fps

FFMPEG::open_decoder: some stream times estimated: /run/media/terje/Videoklipp/Cineform/opus.webm
audio0 pad 16 0 (16)
FFMPEG::open_decoder: some stream times estimated: /run/media/terje/Videoklipp/Cineform/opus.webm
Total excess of backups: -50
Session time: 0:01:49
Cpu time: user: 0:00:15.458 sys: 0:00:02.457