With 'someField': { $exists: true } Mongoose returns a document for which someField doesn't exist

Something strange is happening to me. I run the following Typescript code:

for await (const expression of Expression.find({'definiton': { $exists: true }}))
{
 console.log(Utils.stringize(expression))
}

And the logging returns me the following document, in which the field definition doesn't exist:

{
  "_id": "63cc466d2c2338ef26205618",
  "labels": [
    "⊥"
  ],
  "author": {
    "userId": "63cc466c2c2338ef26205609",
    "username": "webmaster"
  },
  "timestampCreation": "2023-01-21T20:09:17.027Z",
  "tags": [
    "native symbol",
    "logic",
    "mathematics",
    "false"
  ],
  "nbrViews": 1,
  "nbrUsesInExpressions": 2,
  "nbrUsesInTruths": 0,
  "idsUsedExpressions": [],
  "idsUsedExtraAxioms": [],
  "type": {
    "sort": "P"
  },
  "parameters": [],
  "__v": 0
}

If it can be of some help, here is the Mongoose schema of the collection:

const expressionSchema = new Schema<IExpression>({
  labels: {
    required: true,
    type: [String],
    //unique: false,
    trim: true
  },
  author: {
    type: Schema.Types.Mixed, //Author,
    required: true,
  },
  //timestampCreation: Date,
  timestampCreation: {
    type: Date,
    required: true,
  },
  tags: {
    required: true,
    type: [String],
    trim: true
  },
  nbrViews: {
    required: true,
    type: Number,
  },
  nbrUsesInExpressions: {
    required: true,
    type: Number,
  },
  nbrUsesInTruths: {
    required: true,
    type: Number,
  },
  idsUsedExpressions: {
    type: [Schema.Types.ObjectId],
  },
  idsUsedExtraAxioms: {
    required: true,
    type: [Schema.Types.ObjectId],
  },
  type: {
    required: true,
    type: Schema.Types.Mixed //Sort|UnknownType|CompoundType
  },
  parameters: {
    type: Array
  },
  definition: {
    type: Schema.Types.Mixed //StatementBoundVariables,
  },
  idInstantiatedTruth: {
    type: Schema.Types.ObjectId,
  },
  idResultingTruth: {
    type: Schema.Types.ObjectId,
  }
})

And I've tried with 'definiton': { $ne: null } and Expression.find().where('definiton').exists(true)), I've removed and added some quotes, but the result remains the same.

Anyone to understand this?

Your spelling of 'definition' is inconsistent.

In your schema file you correctly define it as 'definition' but in your example query you use 'definiton' ( note the missing 'i' before the last two characters )