I finally found the issue, it is related to the timestamp (score) when the user get registered.
I did not realized that my import was generating wrong type of time stamp {score":{"$numberLong":"1454992330000"}} rather than {score":1.509408313172e+12}
I finally run this fixup to cope with my issue:
db.objects.find({_key:"group:registered-users:members"}).forEach(
function(e){
db.objects.update(
{"_id": e._id},
{"$set": {"score": 0+e.score}}
)
}
);
I hate this mongojs thingy