Some news, it seems that .position.set() work on the code sandbox and not in my real code 
FYI the code is there : personal-website/script.js at 41628a21ed2aa148be4e21179a632e17fa949062 · bouteillerAlan/personal-website · GitHub
If I found why I tell you and close the conv ^^
EDIT : ok, I redo the code and everything work fine now, I don’t know why…