Mongoose Validation Error Cannot read properties of undefined (reading 'options')

Working with Data Drivers
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I have several users who I cannot manage to call the save() function from mongoose on. The users are established and have been fine for a few years. I can update them just find in Atlas or Compass, or by calling findOneAndUpdate. However any time I try to save() or validate() the user after fetching it from the DB using mongoose’s findOne function, I get the following error.

{
  "errors": {
    "undefined": {
      "name": "ValidatorError",
      "message": "Cannot read properties of undefined (reading 'options')",
      "properties": {
        "message": "Cannot read properties of undefined (reading 'options')",
        "type": "cast",
        "reason": {}
      },
      "kind": "cast",
      "reason": {}
    }
  },
  "_message": "USER_MODEL validation failed",
  "name": "ValidationError",
  "message": "USER_MODEL validation failed: undefined: Cannot read properties of undefined (reading 'options')"
}

I can see from the error that it’s something about Type Casting, however I’m not sure which property is even the issue.

tried to figure out which property was failing to cast by calling validate() on the user and passing a specific property to validate instead of the whole user.
If I fetch the user, and loop through all properties and try to validate(), whichever property was first in the loop fails to validate, and the rest pass. I confirmed this by sorting the properties in a different order and trying again. The validation still failed on the first property.

I also tried validating each property individually, but instead of doing a findOne once at the beginning before the loop, I fetched the user in the loop each time. Doing it this way, EVERY property failed to validate individually.

I’ve gone into atlas and compared these users to a user who has no issues saving or validating and cannot find any data differences / discrepancies.

My question is, has anyone ever encountered this? How did you find the error and if possible how did you solve it?