I have a computer that exhibits the following behavior.
If I install Intel 10th gen cpu G6400: I can enable CSM, and then see options to boot USB from legacy / UEFI
If I install Intel 11th gen cpu i9-11900:
CSM still appears in menu, and you can try to enable it, but upon boot CSM reverts back to disabled, and options to boot USB from legacy / UEFI never appear.
Based on what you are saying, this is a bug in the manufacturer's BIOS, and NOT any difference in the Intel 11th gen chip vs the previous generation.
Are you 100% sure this is correct, or is it possible that the 11th gen i9-11900 has some kind of change that makes switching CSM on and booting from legacy USB impossible?