Image mirroring fails

**Describe the bug**
Hello when mirroring image of
it fails with error

```
2026/07/27 18:21:17 [ERROR] : [Worker] error mirroring image docker://registry.connect.redhat.com/intel/intel-dsa-plugin@sha256:18b1cd603a57255ac387ea056ef5d96f325d59eb66ce78c3cc0fa4f5c0534b6c (Operator bundles: [intel-device-plugins-operator.v0.36.0] - Operators: [intel-device-plugins-operator]) error: reading signatures: reading manifest sha256-18b1cd603a57255ac387ea056ef5d96f325d59eb66ce78c3cc0fa4f5c0534b6c.sig in registry.connect.redhat.com/intel/intel-dsa-plugin: name unknown: Image not found
....
2026/07/27 18:21:17 [ERROR] : [Worker] error mirroring image docker://registry.connect.redhat.com/intel/intel-gpu-plugin@sha256:2569cfa01f54d7f73acc889c380bc2f7f5a3098866b9b43b48ae2fa9fa34355c (Operator bundles: [intel-device-plugins-operator.v0.36.0] - Operators: [intel-device-plugins-operator]) error: reading signatures: reading manifest sha256-2569cfa01f54d7f73acc889c380bc2f7f5a3098866b9b43b48ae2fa9fa34355c.sig in registry.connect.redhat.com/intel/intel-gpu-plugin: name unknown: Image not found
```

**To Reproduce**

set a minfest file with mirroring the plugin operator:
```yaml
apiVersion: mirror.openshift.io/v2alpha1

apiVersion: mirror.openshift.io/v2alpha1
kind: ImageSetConfiguration
archiveSize: 10
mirror:
operators:
- catalog: registry.redhat.io/redhat/certified-operator-index:v4.19
packages:
- name: intel-device-plugins-operator
channels:
- name: stable
- name: sriov-fec
channels:
- name: stable
#minVersion: v2.9.0
#maxVersion: v2.9.0

```

run oc mirror

```
oc-mirror -c imageset-config-intel.yaml --remove-signatures=false --secure-policy=true --cache-dir ./mirror-cache/ file://./mirror-tar-4226
```
[ if you set the flags to these it will succeed: --remove-signatures=true --secure-policy=false )

you see the above error.

**Expected behavior**
expect to mirror the operator to the tar file with no issue

**Screenshots**
N/A

**System (please complete the following information):**
oc-mirror version 4.20
Linux any.

**Additional context**
Add any other context about the problem here.

Start with the provided ImageSetConfiguration and reproduce the failure using the oc-mirror command with signature preservation and secure policy enabled. Compare it with the succeeding flag combination and trace the signature lookup for the Intel plugin images; done means the operator mirrors to the tar file without requiring signature removal.

Written by the indexing model from the issue text.