Certified operator index is complicated when it comes to image signatures. oc mirror 4.21 started to enforce signature pulls. Either use oc mirror 4.20.x binary or use the below workaround.
mkdir registries.d
cat <<EOF > registries.d/redhat.yaml
docker:
registry.connect.redhat.com:
use-sigstore-attachments: false
EOFoc-mirror --v2 --authfile ~/.docker/config.json --registries.d "${PWD}/registries.d" --config ./imageset-config.yaml file://${PWD}/mirror