Spider mites, drought and soggy soil conditions are the three most likely causes. Since you have dealt with these possibilities. If the foliage totally browns in an area it will not return on those branches but other parts of the tree will continue to grow.
Try to keep the soil evenly moist. Time will tell if it can turn around and recover or not but I would not give up on it just yet. I think that the comment about it having a 50% survival rate is not correct. With proper care the plants should survive just fine. Make sure the plant is getting lots of sun. as they don't like shade.
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