Rsync 3.1.2 and High Sierra sparse bundle problem
Driving today, I realized I had not reset the ACL for that folder; part of my upgrade maintenance ended up involving some group assignment repairs, and I had not changed that group, nor promulgated the new group, so an unassigned unique identifier was listed as the ACL. Rsync does not like this.

Once I assigned workgroup back to this folder, and promulgated the ACL, rsync 3.1.2 handled the job correctly.