Meringue is hygroscopic, that is, it actively absorbs moisture from the atmosphere. So, as the pastry and lemon filling are damp, the meringue is going to go soft and collapse no matter what.
Think about making meringues, the cakes, not the pie topping, you whip them up and dry them out for hours and hours in a very slow oven. Once they are cooked you have to put then in a bone dry airtight box and only sandwich them with cream or whatever just before serving, otherwise they go soft.
Soft is the normal nature of meringue.