thx for your answers/advises.
i re-unwrapped my object, following various techniques from many tuto ^^ (mainly this one : https://www.youtube.com/watch?v=VyTH0Xtn_D4)
uvs seem clean and fine to me (every islands have at least 3px padding from each others). no more black face (yes !), but as you can see below, shadow bleeding problem occurs
i tried different uvs layouts/templates (i don’t know how you call it), it doesn’t solve.
but magic happens when i turn light from static to movable…everything becomes as it should be, clean and smooth.
i’m ok with that for now, but from what i know, movable lights “cost” a way more than static, so it might become a problem when my level will grow up. so, again, if you have any suggestion…