Why would push notifications stop working for iOS but not android?

Question

We have a mobile app built in Angular/Ionic that all of a sudden stopped receiving push notifications on iOS, however they still work on android.

the message we get back from firebase is this:

{
  "error": {
    "code": 400,
    "message": "The registration token is not a valid FCM registration token",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.firebase.fcm.v1.FcmError",
        "errorCode": "INVALID_ARGUMENT"
      }
    ]
  }
}