The corky bark is fragile and if mishandled while repotting, etc the bark on the lower trunk can get rubbed off and the tree appears to have inverse taper

Corky bark close to the ground is also prone to rot where it stays damp. Again reverse taper but stands out even more and takes a lot more effort to restore.
Although the pot says this tree is corky bark variety I'm not so sure. I think there should be some sign of the bark starting to thicken by now. Also the leaves and branching just don't look quite right for corky variety. Still a tree with great bonsai potential though.