iPad Pro 11 inch battery draining

My new iPad Pro 11 inch worked perfectly well for five months, then the battery started draining from 100% to 1% overnight when it wasn’t used and several times switched off from the side. I went to Apple three times and they eventually filled changed it three weeks ago. However three times in the last week it has done the same thing. 100% to 1% overnight. Not impressed. Why is it doing this. The old iPad Pro 11 inch never did this.