I learned more mature trees (3/4" - 1" trunks) can get this from being covered when I cut circles of panda film to cover my 10 gallon pots (coco) to reduce evaporation. I just made an X in the middle and a slot from the center to the edge so I could place the circle around the base of the stem. The X cut flaps would fold up covering the stem base.
After several days like this and the coco never being very dry, I noticed the base of the stem getting a little funky, sorta pale and sickly where the flaps covered it, otherwise roots and the rest of the plant were extremely healthy. I cut the little flaps off so the hole was square and left a little bit of room around the stem base and the problem was fixed.