Update: So I was looking about iron chlorosis, too low soil acidity/low pH (though it has been potted, we live in a place with *extremely* alkaline soil and I don't know if potted soil comes from the same area), and zinc deficiency. I actually think the zinc deficiency symptoms look quite similar, and iron chlorosis seemed similar enough that it actually got me looking at those types of things. I wonder if I overfertilized it at some point and that exacerbated its zinc deficiency? Any thoughts on those? I also read that too low soil acidity can prevent the plant from absorbing key nutrients? Thanks again.

You can never treat a containerized plant as if it were part of nature. Ever. These plants get every single bit of their water and nutritional needs through the attention and care that we choose to give them.

The potting mix it came in and the stuff you've recently transplanted it in have absolutely nothing whatsoever to do with your native (outdoor) soil. I PRESUME that you used a potting medium rather than a bucketful of your garden soil! ;-)

Pre-packaged potting mixes are generally pH ready and adjusted without you having to add anything to raise or lower the acidity/alkalinity. They tend to run just a tad on the acid side, which is preferable.

Well the treating the plant as if it were nature only extended to not being completely regular with the watering... and it had never been a serious issue until recently.
Of course I used potting soil, but I don't know what they do to it and just wanted anyone who had any ideas to have the full story.
I ultimately decided that it was probably a mineral deficiency (particularly zinc but maybe some others like iron), so the next day I went and bought a chelated iron/zinc/copper/magnesium mix and sprayed it on the branches and have watered it with it twice, once that day and once just now. Just after the first application, the dieback slowed down dramatically. I just found that one more branch has died but there is quite a bit of new growth on the plant overall (no fullfledged leaves yet), so I'm hopeful. Just thought anyone else who was running into similar problems in the future might appreciate the info.

Sounds like the saturated lower layers of the soil have led to cyclical death and regrowth
of roots, which ultimately resulted in a paltry root-system (at the time of inspection).
With the lack of water, fine roots most likely died.

Monitor the soil-moisture with a wooden skewer or chop-stick, and be careful not to overwater.
If the soil stays wet for a long time, please consider re-potting between growth flushes in early Summer.

Josh

Right, I've always been careful not to overwater, as that is what I've heard was the big problem. Thanks for the skewer/chopstick monitoring idea, I'll definitely do that and use it to monitor the water levels and see if I need to completely repot it. I really think the zinc thing was a huge, huge problem (it just went down the checklist of symptoms and started recovering after the zinc applications). It's still a bit delicate but I think it's getting better quickly.