running this:
ffmpeg -loglevel info -y -i bdf2423966febd7d451799e4980507689.mkv -map 0:3 -c:s webvtt -hls_time 10 -hls_playlist_type vod -hls_segment_filename subtitle_3_%04d.vtt subtitle_3.m3u8

i get:
    Stream #0:0(ger): Video: h264 (High), yuv420p(progressive), 1920x1080, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default) (forced)
    Stream #0:1(ger): Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s (forced)
    Stream #0:2(eng): Audio: dts (DTS), 48000 Hz, 5.1(side), fltp, 1536 kb/s
    Stream #0:3(eng): Subtitle: subrip
[mpegts @ 0x75d470] No streams to mux were specified
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argumentStream mapping:
  Stream #0:3 -> #0:0 (subrip (srt) -> webvtt (native))
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