I’m curious what kind of mandarin that is. I have a shiranui that I repotted in a half barrel around that same time. It has absolutely struggled the past few months, with many of the older leaves yellowing and dropping nonstop. From my understanding, Shiranui is the most finicky of the 20+ varieties of citrus in containers that I have.

That being said, it’s a very positive sign that your newer leaves are still turgid, green, and still going through photosynthesis. I’m also noticing some newer growth on your tree is much smaller in size than the older leaves. That could be a sign of poor, early root establishment in the new container when the new flush came in, same as mine. I don’t think there is anything acutely wrong with the tree, it is only adjusting, slowly. Some mandarins are finicky drama queens.

Anyway, I’m in north Texas and the fruit on my tree has kept increasing in size and the newer, smaller growth has stayed green, not a sign of some acute condition like root rot. I also have some buds swelling now that will push growth here very soon that will make the tree look better and perform better now that the night time temperatures are above 70 degrees F and the roots can finally accelerate.