VueJs adding new key to object reference issue
You are breaking reactivity. You will break reactivity if you modify a property that was not declared when you mount the component. You're trying to directly modify objects in an array that were added later and Vue cannot track that. Because of this, I require all programmers on our team to use this.$set whenever they modify a property on an object.
You can replace:
this.rows[rowid][fieldName] = value;with
this.$set(this.rows[rowid], fieldName, value)