Hi Pawel,
I might have missed something, for me the boxes are still showing checked when I check in my dashboard. Before you check in the dashboard, can you view (not sandbox view) the dashboard and check all the boxes, and go back to edit mode and check in. Will this keep the boxes checked when you check in your dashboard?
I also have the script for you to check all the boxes in case the above doesn’t work for you. You can add something similar on the Dashboard Ready event:
//get all the items from the checkbox.
//(you many have a different parameterCheckboxList name. Update your parameterCheckboxList name accordingly)
var items = parameterCheckboxList1.control.items;
//loop through the checkbox items and set them to checked
items.forEach(function(item){
item.value = true;
});