Thanks to you I finally find out how to resolve the issue.
It was linked with the ffmpeg file provided by default with the QNAP OS that was not handling properly the AAC.
So I had to provide an updated ffmpeg file working with the Intel CPU on my QNAP, and then restart the roon core.
Website to get the ffmep file: John Van Sickle - FFmpeg Static Builds
Procedure to update the FFMEP is accessible when opening the roon core on QNAP → Click on the ffmpeg button
Related topic on roonlab community: Qnap AAC bug is back with the latest update - #28 by Chrislayeruk