Container format requires global headers
This issue was fixed with commit 955b818:

commit 955b818cf947473ec94a3fe8aa7f408b119fbbc9
Author: Clément Bœsch &lt;clement@stupeflix.com&gt;
Date:   Thu Sep 1 16:48:45 2016 +0200

    ffmpeg: switch to codecpar


You can suppress the warning on older version like so:


ffmpeg -i Miaow-02-Hidden.m4a -c copy -flags global_header outfile.m4a


Example