it's a problem from current retroarch version
No, since the problem doesn't show up on other platform.
it's a problem from the core when it's build for arm (probably this one)
Unlikely, since the core on RetroPie works with Netplay using the latest RetroArch version.
The issue in this post is related to missing .info files, which were not part of Retropie's RetroArch installation until 1 year ago. Users which upgraded their RetroArch package without updating the RetroPie-Setup script had this kind of errors, which are solved with an update of RetroPie-Setup and then re-installing RetroArch.
I don't think your issue is related to the current topic.