Cannot change concurrency mode from OPTIMISTIC_WITH_ENTITY_GROUPS to OPTIMISTIC (500 response)

Google Cloud Database
1

I manage a project that started on App Engine v1 and our database has now been migrated to Firestore in Datastore Mode (overall extremely happy). However, I’m trying to change the concurrency mode of one of my test projects from OPTIMISTIC_WITH_ENTITY_GROUPS to OPTIMISTIC. When I run gcloud firestore databases update --project=<my project> --concurrency-mode=optimistic the request receives a 500 response:

ERROR: (gcloud.firestore.databases.update) HttpError accessing <https://firestore.googleapis.com/v1/projects/<my project>/databases/(default)?alt=json&updateMask=concurrencyMode>: response: <{'x-debug-tracking-id': '9948352006159926144;o=0', 'vary': 'Origin, X-Origin, Referer', 'content-type': 'application/json; charset=UTF-8', 'content-encoding': 'gzip', 'date': 'Thu, 30 Apr 2026 19:55:35 GMT', 'server': 'ESF', 'x-xss-protection': '0', 'x-frame-options': 'SAMEORIGIN', 'x-content-type-options': 'nosniff', 'alt-svc': 'h3=":443"; ma=2592000,h3-29=":443"; ma=2592000', 'transfer-encoding': 'chunked', 'status': 500}>, content <{
  "error": {
    "code": 500,
    "message": "Internal error encountered.",
    "status": "INTERNAL"
  }
}

Is there anything I need to do to prepare my instance for this change? What am I doing wrong here?