With an ImageSet like this:

kind: ImageSetConfiguration
apiVersion: mirror.openshift.io/v1alpha2
storageConfig:
  local:
    path: ./oc-mirror-metadata
mirror:
  platform:
    graph: true
    architectures:
      - amd64
    channels:
    - name: stable-4.12
      minVersion: '4.12.28'
      maxVersion: '4.12.28'
      shortestPath: true
      type: ocp
    - name: eus-4.14
      minVersion: '4.12.28'
      maxVersion: '4.14.15'
      shortestPath: true
      type: ocp

oc mirror only mirrors 4.12.28 and 4.14.15, but it doesn't migrate any valid intermediate 4.13.z version, so an eus upgrade cannot be performed. 

EUS upgrades require the control plane to go through the intermediate versions, i.e. for a 4.12 to 4.14 EUS upgrade, a 4.13 version must be available so that the user can do the 4.12-->4.13-->4.14 upgrade (actually, "EUS upgrades" should have been named "upgrades with the non-master MCPs paused", because they are not much more than that).

# oc mirror version --output yaml
Logging to .oc-mirror.log
clientVersion:
  buildDate: "2024-02-08T23:49:33Z"
  compiler: gc
  gitCommit: 68747309c90d1c64d5404ec9ea12bc7c36a80555
  gitTreeState: clean
  gitVersion: 4.14.0-202402081809.p0.g6874730.assembly.stream.el8-6874730
  goVersion: go1.20.12 X:strictfipsruntime
  major: ""
  minor: ""
  platform: linux/amd64

Always

    1. Create an ImageSetConfiguration as per description (with shortestPath)
    2. Run oc mirror
    3. (Optional) Create the updateService in OSUS

No intermediate 4.13 version mirrored. EUS upgrade not possible.

The required minimum 4.13 version(s) mirrored so that an EUS upgrade between minVersion and maxVersion on eus-4.14 channel is possible

A suitable workaround is to include the stable-4.13 channel on the ImageSetConfiguration, with the valid 4.13 version(s) in minVersion and maxVersion. Even if those versions are mentioned only in stable-4.13 channel, they also appear in the eus-4.14 channel in the update graph, so the EUS upgrade becomes possible.

For that to work with OSUS, it was also needed to apply the latest updateservice generated by oc-mirror.