[Vuejs]-Vuex store – pushing item in state to a state array causes the previous item in the array to change to the current state item

That’s because of the nature of Vuex, you have only and only one copy of the data inside your state object, so pushing state.todo to the array pushes the only copy of todo that you have in your state, so when you update it again it changes the reference inside the array, you need to push a new element to your state, you need to receive the title in the "addTodo" method and create and push the new object:

addTodo(state, t) {
  state.todos.push({title: t});
}