@slbaumgartner - yes, I know what you mean and I have been very careful. I think we have now established that this is a bug which can be replicated.
I was actually not wanting to turn it - but it did, even though it was on the same surface. If it happens again, I’ll make a video which illustrates this better.
It’s not a bug. SketchUp can copy a single face (or void) and even more than one that are on the same large face. Even to another face in a different plane, aligning to that face if needed.
What you try to do is copy two voids on different faces (one on top and on on the bottom of the shelf). SketchUp doesn’t handle two “glue to / cutting opening” chunks of geometry that are on different faces in just one operation.
A hole that doesn’t go through the entire shelf actually has only one void on one face.