Hi Miles Wada,
Let me kindly explain that we tested this problem under the same conditions:
Using a Surface Pro, Copilot+ PC with Snapdragon X Elite(12Core) 16GB RAM,512GB SSD.
Also tested is the Surface Pro Signature Keyboard with Slim Pen 2- (Platinum).
The results of the test are that the keyboard can work on the Surface Pro, Copilot+ PC (Surface Pro 11), so the official compatibility information is correct.
Therefore, if your keyboard doesn't currently work properly in UEFI, what you need to do is send it to the Surface support team for inspection and repair.
Or there's another way to test it. If you are near a Microsoft sales store, you can bring your Surface and keyboard cover to the store, tell the clerk you are here, and test with the store's equipment. More test samples will help you determine whether the problem is from the keyboard you are using or from your Surface.
If you notice the problem reoccurring while testing with a new keyboard, or a repaired keyboard, you may want to send your Surface to the Surface support team for inspection as well.
Therefore, the most complete way is to send the Surface together with the keyboard to the Surface technical support team for testing.
Best Regards,
Mitchell | Microsoft Community Support Specialist