I'm having a similar issue. Xcode will display the .scn file but as soon as I click on the object the display starts cutting out then finally just goes black. I did what you said and opened Xcode using Rosetta. That fixed the .scn display issue but now when I open it using Rosetta the main.storyboard view controllers are black and wont let me edit them.
xcode 12.3
big sure 11.1
M1 air 8/256
Tyson151
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Tempe, AZ, United States