Can't save NumberArray

I'm using KeystonJS and I'm trying to update a NumberArray by index, but when I save the change, it doesn't save it in the admin UI.

Here's my code :

Project.model.findOne({projectId: projectId}).exec(function(err, project){
    if (err) return res.apiResponse({statusCode: 500, message: err});
    if (!project) return res.apiResponse({ statusCode: 400, message: "Can't find this projects" });
    console.log("BEFORE : " + project.detailsPerc[index]);
    project.detailsPerc[index] += 40;
    // project.progressionPerc += 10;

    project.save(function(err, result) {
        if(err) {
            console.log(err);
            res.apiResponse({
                statusCode: 400,
                items: err
            });
        }
        else {
            console.log("RESULTAT :" + result);
            console.log("AFTER : " + project.detailsPerc[index]);
            res.apiResponse({
                statusCode: 200,
                items: project
            });
        }
    })
})

The result is good :

RESULTAT :{ _id: 5ab158fb87475e062c0122c9,
clientId: '1',
__v: 2,
client: 5ad5f4aaa42faf03b95cacf6,
detailsPerc: [ 40, 0, 0 ],
details: [ 'Masterisation', 'EMM', 'Developpement' ],
progressionPerc: 20,
projectId: '1',
name: 'jarviis' }

But yet when I go in the admin UI, my array still show me [0,0,0]. Also, it works when I update project.progressionPerc.

Does anyone know what I'm doing wrong? Thanks guys!

Software	Version
Keystone	4.0.0-beta5
Node	v9.8.0