Sorry, this doesn't make sense to me.
Any human contributor can also plagiarize closed source code they have access to. And they cannot "transfer" said code to an open source project as they do not own it. So it's not clear what "elephant in the room" you are highlighting that is unique to A.I. The copyrightability isn't the issue as an open source project can never obtain copyright of plagiarized code regardless of whether the person who contributed it is human or an A.I.
Human beings can create copyrightable code.
As per the US Copyright Office, LLMs can never create copyrightable code.
Humans can create copyrightable code from LLM output if they use their human creativity to significantly modify the output.
a human can still be held accountable though, github copilot running amock less so