Following the steps in the tutorial fixed my auto reconnect issue. Thanks.

@substring thank you again for the quick reply.

RESOLVED

@meugster kein Problem, dafür gibt es ja auch ein Forum.. 😉

News :

I have tested to analyse the audio signal from my amp :

On the first boot it says "Signal PCM" on both ES and Retroarch running a game, I can hear the sound OK for both.

After 1 or some reboot (randomly), still says PCM on ES, but says "Unknown" signal on retroarch running a game :

Screenshots :
https://imgur.com/a/Hd6cY

@oyyodams
Du coup, j'ai réussi.
En fait, j'ai réinstallé une version d'une image de recallbox, avec Etcher.

Make sure you start the pad by pressing R + START.

Mine take several minutes for the connection to be established; if the blue led turns off, start the pad again (same thing if the pad connects only within seconds: it's probably a bad sign =))