Hi,
Sorry to hear you've received this email. We're are in a similar situation except that we didn't do any of what you said you had done. Although we did experience some drop in downloads, we can't figure out if we had anything to do with it. Same app, same content.
In relation to the message, did they ever come back to you with an answer? Did you ever see this message in your App Store Connect dashboard or developer page?
I appreciate your response
Thank you