We don't see any obvious causes of the bark loss, and given how good the canopy looks, we agree the tree is probably not in any danger at present. Flaking bark loss on several tree species is normal as they age, though usually not to this depth and not this localized when it's a trait inherent to the species. Wild animals can occasionally remove tree bark as well, and Katsura trunks naturally do develop loose-ended plates of bark that would be easier to cleave off. An on-site assessment from an arborist would be more thorough, and if you'd like a second/third opinion, we suggest you have a certified arborist inspect the tree (again); you can use a different person/company if your first specialist was also an arborist. Some arborists are employees of tree-care companies while others consult independently.

Miri