AutoCAD 2018 for MAC has not been released yet, it's on a fall/winter release cycle where the world of Windows is on a spring release cycle.

It's been like that since the dawn of the Mac versions.

For most of us users in business with project teams outside of our office, we know to always save down to help each other out. Sometimes we also avoid upgrading to format-changing versions of AutoCAD until most of us have caught up as well.

See tip from an Autodesk team member that posted after I did: it appears the web-based AutoCAD-ish variant might help you out as a fix. Good to know.