Comment 6 for bug 1439549

a workaround is easy:

while true; do
  mirscreencast -m /run/mir_socket --stdout --cap-interval 1 -s 270 480 -n 60 >> /tmp/video
  printf "."
done

i.a. capture always 60 frames and restart mirscreencast in a loop;

the resulting file /tmp/video plays fine with

$ mplayer -demuxer rawvideo -rawvideo fps=60:w=270:h=480:format=rgba video