Why ffmpeg command swallow stdout

I want to generate thumbnails using ffmpeg, but it seems there exists a bug.

I have written the below code to make this bug reproducible.

#!/usr/bin/env bash

## setup
lab_home="/tmp/playground"

\mkdir -p "$lab_home" && \cd "$lab_home"

\ffmpeg -f lavfi -i testsrc -t 30 -pix_fmt yuv420p -loglevel quiet sample-01.mp4
\ffmpeg -f lavfi -i testsrc -t 30 -pix_fmt yuv420p -loglevel quiet sample-02.mp4

## bug
\find "$lab_home" -type f -name "*.mp4" -print0 |\
while read -r -d $'\0' video; do
  \echo "$video"

  \ffmpeg -i "$video"     \
          -y -f rawvideo  \
          -pix_fmt bgra   \
          -c:v rawvideo   \
          -frames:v 1     \
          -loglevel quiet \
          -vf "scale=iw*min(1\,min(192/iw\,108/ih)):-2,pad=192:108:(192-iw)/2:(108-ih)/2:color=0x00000000" \
          "${video}.thumbnail"
done

## clean
#\rm -rf sample-01.mp4 sample-02.mp4 sample-01.mp4.thumbnail sample-02.mp4.thumbnail

why the output is:

/tmp/playground/sample-02.mp4
tmp/playground/sample-01.mp4

instead of

/tmp/playground/sample-02.mp4
/tmp/playground/sample-01.mp4

Remove or comment out the ffmpeg command work as expected.