On my system (Kubuntu 20.04) man/troff produces errors when its output is redirected:

  $ man ffmpeg-all | wc -l
  troff: <standard input>:6148: warning [p 42, 3.5i, div 'an-div', 0.0i]: can't break line
  troff: <standard input>:6150: warning [p 42, 3.7i, div 'an-div', 0.0i]: can't break line
  troff: <standard input>:11396: warning [p 68, 8.7i]: can't break line
  troff: <standard input>:11398: warning [p 68, 9.2i]: can't break line
  troff: <standard input>:13562: warning [p 79, 6.0i]: can't break line
  troff: <standard input>:13564: warning [p 79, 6.5i]: can't break line
  troff: <standard input>:26366: warning [p 132, 17.2i]: can't break line
  troff: <standard input>:38313: warning [p 174, 27.5i]: can't break line
  33160

No idea what it means.