af-command succeeds in mp.add_key_binding(), but it fails in mp.register_event()

[jmaibaum]

I am trying to send an af-command to a (custom) libav filter [1] during/after "seek" events via mp.register_event(). This doesn't work, I am getting error running command. When I am sending the exact same af-command via a key-binding, it succeeds. Why doesn't it work during "seek" events? Is it possible to make it work? I couldn't find anything in the docs that explains this different behaviour.

mpv/ffmpeg version:

mpv v0.39.0-444-g1f1c30146c Copyright © 2000-2024 mpv/MPlayer/mplayer2 projects
 built on Dec  1 2024 20:18:05
libplacebo version: v7.349.0
FFmpeg version: git-2024-11-30-3b14d12042
FFmpeg library versions:
   libavcodec      61.26.100
   libavdevice     61.4.100
   libavfilter     10.6.101
   libavformat     61.9.100
   libavutil       59.47.101
   libswresample   5.4.100
   libswscale      8.12.100

mpv command:

mpv /path/to/video.mp4 '--af=@timecode:lavfi="ltcsrc@my[ltc],[aid1][ltc]amix=weights=0 1"' --script=resync-timecode.lua

Contents of resync-timecode.lua

function resync_timecode()
    pos = mp.get_property("time-pos")
    print(pos)
    res, error = mp.command_native({
        name = "af-command",
        label = "timecode",
        command = "start_timecode",
        argument = tostring(pos),
	target = "ltcsrc@my",
    })
    print(res, error)
end

mp.add_key_binding("a", "resync-tc", resync_timecode)
mp.register_event("seek", resync_timecode)

mpv output

The 3rd and 4th line starting with [resync_timecode] were invoked by pressing "a" ~3s after the "seek" event.

[lavf] queuing seek to 1609.394918
[lavf] execute seek (to 1609.394918 flags 32)
[lavf] seek done
[resync_timecode] 1609.394918
[resync_timecode] nil error running command
[resync_timecode] 1612.694417
[resync_timecode] nil nil

[1]: Code of said libav filter is here: https://git.sr.ht/~jmaibaum/ffmpeg/refs/ltc It is a LTC (linear time code) source based on https://github.com/x42/libltc