What happens?

Given a valid base64url-encoded value, the current pattern in OpenAPI enforces a validation error. Example: https://app.swaggerhub.com/apis/Plattform_i40/DiscoveryServiceSpecification/V3.0.1_SSP-001#/Asset%20Administration%20Shell%20Basic%20Discovery%20API/GetAllAssetAdministrationShellIdsByAssetLink
Query parameter assetIds:

{
  "name": "serial-number",
  "value": "0123456",
  "externalSubjectId":
    {
      "keys":
        [
          {
            "type": "AssetAdministrationShell",
            "value": "defaultSpecificAssetId",
          },
        ],
      "type": "ModelReference",
    },
  "semanticId":
    {
      "keys":
        [{ "type": "GlobalReference", "value": "eClassDefaultSemanticId" }],
      "type": "ExternalReference",
     }
}

is correctly base64url encoded as
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

However, a server strictly validating against the OpenAPI file will throw a validation error ("invalid encoding" or similar).

This value will however pass:
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

Why is this wrong?

      schema:
        type: array
        items:
          type: string
          format: byte

means base64 encoding, not base64url.

How should it be fixed?

As base64url can not be expressed in OpenAPI, remove the format: byte from all files:

      schema:
        type: array
        items:
          type: string

Compliance to base64url must be described/checked outside of OpenAPI anyway.

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