Confirm that the amdgpu firmware files are actually in the initramfs image. E.g.
$ sudo lsinitcpio /boot/initramfs-6.6-x86_64.img | grep 'amdgpu/psp_13_0_0_*'
usr/lib/firmware/amdgpu/psp_13_0_0_sos.bin
usr/lib/firmware/amdgpu/psp_13_0_0_ta.bin
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