See the strange shadow buffer like rectangle in Rendered view - only visible when viewing from above.
Apologies for the delay in reply, I believe this was caused by a bug fixed back in Oct. of 2019 to do with the self shadowing slider in the display mode settings but that was specifically to do with technical modes…
https://mcneel.myjetbrains.com/youtrack/issue/RH-42875
I don’t see this problem with sun shadows on a shadows only ground plane in the Rhino 6 for Mac Beta. Let me know if you still have an issue.