As in the topic - Yii tries to save attribute that is abstract thus throwing an error. There is not much going on in the code, I have added it to attributes() and created getter and setter
    public function attributes() {
        return array_merge(
            parent::attributes(),
            ['fliLabel']
        );
    }
    
    public function setFliLabel($value) {
        $this->fliLabel = $value;
    }
    public function getFliLabel() {
        return $this->fliLabel;
    }
It is even more confusing as in this issue Qiang says "QueryBuilder ensures only table columns are being inserted/updated."