Can't remove "encoder" metadata field

[dingolover6969]

Summary of the bug: ffmpeg always leaves "encoder: Lavf57.75.100" in my output files, and I cannot get it not to using the -metadata option. the -metadata option works for any other field.

How to reproduce:

ffmpeg started on 2017-06-29 at 14:34:33
Report written to "ffmpeg-20170629-143433.log"
Command line:
ffmpeg -i "01 - Born Under Punches (The Heat Goes On).flac" -metadata "encoder=" -acodec copy "01 - Born Under Punches (The Heat Goes On)..flac" -report
ffmpeg version N-86669-gc1d1274 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 7.1.0 (GCC)
  configuration: --disable-static --enable-shared --enable-gpl --enable-version3 --enable-cuda --enable-cuvid --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-nvenc --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-zlib
  libavutil      55. 67.100 / 55. 67.100
  libavcodec     57.100.102 / 57.100.102
  libavformat    57. 75.100 / 57. 75.100
  libavdevice    57.  7.100 / 57.  7.100
  libavfilter     6. 94.100 /  6. 94.100
  libswscale      4.  7.101 /  4.  7.101
  libswresample   2.  8.100 /  2.  8.100
  libpostproc    54.  6.100 / 54.  6.100
Splitting the commandline.
Reading option '-i' ... matched as input url with argument '01 - Born Under Punches (The Heat Goes On).flac'.
Reading option '-metadata' ... matched as option 'metadata' (add metadata) with argument 'encoder='.
Reading option '-acodec' ... matched as option 'acodec' (force audio codec ('copy' to copy stream)) with argument 'copy'.
Reading option '01 - Born Under Punches (The Heat Goes On)..flac' ... matched as output url.
Reading option '-report' ... matched as option 'report' (generate a report) with argument '1'.
Finished splitting the commandline.
Parsing a group of options: global .
Applying option report (generate a report) with argument 1.
Successfully parsed a group of options.
Parsing a group of options: input url 01 - Born Under Punches (The Heat Goes On).flac.
Successfully parsed a group of options.
Opening an input file: 01 - Born Under Punches (The Heat Goes On).flac.
[NULL @ 0000000000e3bee0] Opening '01 - Born Under Punches (The Heat Goes On).flac' for reading
[file @ 0000000000e3ce80] Setting default whitelist 'file,crypto'
[flac @ 0000000000e3bee0] Format flac probed with size=2048 and score=100
[flac @ 0000000000e3bee0] Before avformat_find_stream_info() pos: 9095 bytes read:32768 seeks:0 nb_streams:1
[flac @ 0000000000e3bee0] All info found
[flac @ 0000000000e3bee0] After avformat_find_stream_info() pos: 16263 bytes read:32768 seeks:0 frames:1
Input #0, flac, from '01 - Born Under Punches (The Heat Goes On).flac':
  Metadata:
    ALBUM           : Remain in Light (5.1 downmix)
    album_artist    : Talking Heads
    ARTIST          : Talking Heads
    COMMENT         : http://downmix.sharoma.com
    DATE            : 1980
    GENRE           : Post-Punk
    TITLE           : Born Under Punches (The Heat Goes On)
    track           : 01
  Duration: 00:05:49.80, start: 0.000000, bitrate: 838 kb/s
    Stream #0:0, 1, 1/44100: Audio: flac, 44100 Hz, stereo, s16
Successfully opened the file.
Parsing a group of options: output url 01 - Born Under Punches (The Heat Goes On)..flac.
Applying option metadata (add metadata) with argument encoder=.
Applying option acodec (force audio codec ('copy' to copy stream)) with argument copy.
Successfully parsed a group of options.
Opening an output file: 01 - Born Under Punches (The Heat Goes On)..flac.
[file @ 0000000000e40160] Setting default whitelist 'file,crypto'
Successfully opened the file.
Output #0, flac, to '01 - Born Under Punches (The Heat Goes On)..flac':
  Metadata:
    ALBUM           : Remain in Light (5.1 downmix)
    ALBUMARTIST     : Talking Heads
    ARTIST          : Talking Heads
    DESCRIPTION     : http://downmix.sharoma.com
    DATE            : 1980
    GENRE           : Post-Punk
    TITLE           : Born Under Punches (The Heat Goes On)
    TRACKNUMBER     : 01
    encoder         : Lavf57.75.100
    Stream #0:0, 0, 1/44100: Audio: flac, 44100 Hz, stereo, s16
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[NULL @ 0000000000e4cfe0] dropping low score -31 frame header from offset 4795 to 8800
[flac @ 0000000000e3bee0] first_dts 0 not matching first dts 15257600 (pts 15257600, duration 4096) in the queue
No more output streams to write to, finishing.
size=   35801kB time=00:05:49.78 bitrate= 838.5kbits/s speed=5.09e+003x    
video:0kB audio:35792kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.023243%
Input file #0 (01 - Born Under Punches (The Heat Goes On).flac):
  Input stream #0:0 (audio): 3767 packets read (36651297 bytes); 
  Total: 3767 packets (36651297 bytes) demuxed
Output file #0 (01 - Born Under Punches (The Heat Goes On)..flac):
  Output stream #0:0 (audio): 3767 packets muxed (36651297 bytes); 
  Total: 3767 packets (36651297 bytes) muxed
0 frames successfully decoded, 0 decoding errors
[AVIOContext @ 0000000000eb2fe0] Statistics: 2 seeks, 141 writeouts
[AVIOContext @ 0000000000f57040] Statistics: 36660392 bytes read, 0 seeks

[James]

Use -fflags +bitexact as output option.

The downside is that you'll not get metadata like creation_time on containers that support it (mp4 i think), as enabling bitexact output for containers will disable any kind of metadata field that can vary between encodings and ffmpeg builds, but for flac it should be ok.