I'm getting the same issue with Illustrator CS2018 on 10.13.1. I'm saving opened/worked on files onto an exterior drive which is formatted as Mac OS extended (journaled). It's doesn't always happen every time I save and I haven't been able to replicate what triggers it. I doesn't appear to cause a problem with my files other then being annoying extra step.
I'm getting the same issue with Illustrator CS2018 on 10.13.1. I'm saving opened/worked on files onto an exterior drive which is formatted as Mac OS extended (journaled). It's doesn't always happen every time I save and I haven't been able to replicate what triggers it. I doesn't appear to cause a problem with my files other then being annoying extra step.