Is this a laptop? Are you on Qubes 3.x or 4.x?
My USB keyboard has never worked for initial drive decryption. I have to use the built-in laptop keyboard for that. I always assumed it was a security protection because the USB VM is not yet started.
Yeah. Looks like this is a security feature. Look here for reasons why and a possible workaround:
https://github.com/QubesOS/qubes-issues/issues/2270