Random thought: does it display differently depending on whether the page has transparency? Check the transparency blend space, it's going to need to be RGB for reasonably consistent display of Lab colour,
It does! Using the Flattener Preview I found the trqnsparent object. Mystery solved, thank you!
Any transparencies on the page?
Thanks, Eugene – per my reply above, it does, and using the Flattener Preview I found the transparent object. Mystery solved!