A client of mine stumbled across this issue and after some digging it appears to be fairly common. In my experience, FortiClient tends to have more issues with Mac OS in general. For this particular problem though I have had success by rolling back the FortiClient. Downloading the latest from FortiClient tends to be the spot where most people run into issue. Not sure what it is about the older versions that work versus the new one but it is an obvious bug.
If you are sitting around waiting for it to be resolved I wouldn’t get your hopes up. Fortinet tends to be a little slower resolving MAC related issues with the FortiClient software when compared to Windows etc…..guess we can chalk that up to market share.
Anyways, roll back your client to an earlier version and see if that resolves the issue for you. I would give you a specific version to roll to but it seems to vary from environment to environment.
Thing To Remember: Sierra is brand new, so the issues, obviously, may not be on the FortiClient side (at least not completely).