I have tried to invoke the endpoint that publishes the contract with a similar body mentioned below and got a response:

{
  "pacticipantName": "Foo",
  "pacticipantVersionNumber": "dc5eb529230038a4673b8c971395bd2922d8b240",
  "branch": "main",
  "tags": [
    "main"
  ],
  "buildUrl": "<https://ci/builds/1234>",
  "contracts": [
    {
      "consumerName": "Foo",
      "providerName": "Bar",
      "specification": "pact",
      "contentType": "application/json",
      "content": "<base64 encoded JSON pact>"
    }
  ]
}

Response:

{
  "notices": [
    {
      "type": "error",
      "text": "Cannot change the content of the pact for ppe-service-ent-issue-compilation version 1.0.0 and provider ppe-service-ent-issue-compilation, as race conditions will cause unreliable results for can-i-deploy. Each pact must be published with a unique consumer version number. For more information see <https://docs.pact.io/go/versioning>"
    },
    {
      "type": "info",
      "text": ""
    }
  ],
  "errors": {
    "contracts": [
      "Cannot change the content of the pact for ppe-service-ent-issue-compilation version 1.0.0 and provider ppe-service-ent-issue-compilation, as race conditions will cause unreliable results for can-i-deploy. Each pact must be published with a unique consumer version number. For more information see <https://docs.pact.io/go/versioning>"
    ]
  }
}

I dont see any difference in the content that is causing this issue