Stable Diffusion actually has a similar problem. Certain terms that directly call up a particular famous painting by name - say, the Mona Lisa[0] - will just produce that painting, possibly tiled on top of itself, and it won't bother with any of the other keywords or phrases you throw at it.
The underlying problem is that the AI just outright forgets that it's supposed to create novel works when you give it anything resembling the training set data. If it was just that the AI could spit out training set data when you ask for it, I wouldn't be concerned[1], but this could also happen inadvertently. This would mean that anyone using Copilot to write production code would be risking copyright liability. Through the AI they have access to the entire training set, and the AI has a habit of accidentally producing output that's substantially similar to it. Those are the two prongs of a copyright infringement claim right there.
[0] For the record I was trying to get it to draw a picture of the Mona Lisa slapping Yoshikage Kira across the cheek
[1] Anyone using an AI system to "launder" creative works is still infringing copyright. AI does not carve a shiny new loophole in the GPL.