The root of the issue is that UE4 uses the Metal rendering API on Mac and some aspects of Metal are conflicting with Steamworks. Steamwoks will need to be modified to remedy the issue, however it is non-trivial so it has taken time to properly address.
I contacted Valve for an update : They have been hard at work to correct the issue. The majority of the known problems have been corrected and should have the fix in testing sometime in the weeks ahead. Following testing we (Epic) will need to integrate any changes with UE4 which may require additional efforts.
Valve has no official estimate for when the fix will be completed, but be assured it is actively being addressed.
Cheers,
Jonathan