two of my in-ground, in-greenhouse trees have this same issue. One is a Xie Shan satsuma, the other is a Valentine Pummelo. the Valentine Pummelo has had a recurring red spider mite infestation, but the Xie Shan despite being right next to it, hasn't had an obvious mite problem. I spray both when treating, though. These two trees are located right next to each other in an area of my greenhouse that sometimes has standing water when it rains heavily outside, because inside the greenhouse it is actually below grade relative to the outside. I suspect that flooding is the root cause of this for me, and the spider mites are exacerbating it.