OK, we've got this figured out.  The solution was to add:

proxy_set_header  X-Forwarded-For $remote_addr;

to our nginx proxy configuration for this service.  This gets us up and 
running, but somebody who knows the roundup code should figure out what's 
going on here, and especially why this only affects the Change Note 
field.  At a minimum, a note should be added to the documentation 
alerting people to do this if they're running nginx.