Don’t give up just yet. Some of my apples lost all their leaves after turning brown, then they sent up new shoots. I am not sure why this is happening, but it doesn’t seem to kill the tree. It could very well be that the trees have outgrown the pots. With wild apples grown from seed they want to send down a long tap root. They do this so when they are actually wild they can beat the competition to water. If they are grown to long in a pot and outgrow it they may go into a sort of hibernation. Grafted trees have a trait of growing a wide thick root ball because they have ample water all the time and no competition. This is why a nursery can have them for several years.
Good luck, I hope they come back strong for you!