Apps don’t recognize file created by ffmpeg in flutter

I make use of this package in flutter (in Android):

flutter_ffmpeg: ^0.4.2

I run the following script section inside a async function:

final FlutterFFmpeg ffmpeg = FlutterFFmpeg();
await ffmpeg
    .execute(
        "-i /storage/emulated/0/Download/a.mp4 -vn -sn -ac 1 -ar 44100 -ab 32k -acodec libmp3lame /storage/emulated/0/Download/c2.mp3")
    .then((rc) => {print("FFmpeg process exited with rc $rc.")});

Now, the file is created. But the audio player apps just don’t recognize the file. Some apps will even run into an error if I try to play the mp3 file created with them.
Strangely enough, if I move the mp3 file to another folder and back, now all the apps are able to play it, and even recognized them after I send the file to be played. Why isn’t the mp3 file recognized right away (the flutter file system outputs true if I call “file”.exists()).