When using the Auto Scalability feature either through the Editor or through the console command, 'scalability auto', the settings default to Epic quality.
Not that there are Macs out there that cannot handle the Epic settings, but this should not occur on all Macs as their hardware is generally not as powerful as a game development desktop rig.
I noticed that if I created a new blank project from the launcher and set it to Scalable2D/3D the settings upon initial project startup are actually scaled down to High for each scalability setting. Then when I went to select Auto, it defaulted back to Epic. This did not occur on Windows as it immediately defaults to Epic when creating a Scalable2D/3D project (which is expected for this machine).
1. Create new blank project.
2. In the main viewport select Settings > Engine Scalability Settings > Auto.
3. Observe results.
Outcome The editor defaults to Epic quality for all rendering settings.
Expected The editor checks against the hardware and set the scalability settings accordingly.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-44691 in the post.