When this area was excavated the region outside the rock girdle was cut down. It's why I built the rock girdle to begin with because final ground elevation made a weird transition to base of the tree. It's deceiving but mulch elevation is within 1-2" of original ground level. When mulch was applied I always dug the old away from the base of the tree to replace with new. I get what your saying but I was very careful to not allow mulch to build up the trunk knowing that would cause problems. Unless the presence of the mulch in itself caused a specific issue?