Before You File a Bug Report Please Confirm You Have Done The Following...

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
• I have read the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=4.8.4&sourceType=module&code=MYGwhgzhAEAqCmEAuB5ARgK3sJ0DeAsAFDTQAOArmiAJbDQBuYIF8AXNAHYUC2a8AJwDcxAL7FioSDATJ8xUmQE0mSeNDXJ0GaAF4u8AO5xEqTNiQAKAJQiS5ZavWazGAEx6Dx2a4s27CuRUtPQuNhwMAPY0ACby9qRIABY0EAB0LtppTCzq+gCMAAx2pKQA9GUaKemZmG7ZzKyeRSXQ4kSiQA&eslintrc=N4KABGBEBOCuA2BTAzpAXGYUACAXAngA4oDG0AlobgLQrzkB2uA9IdIgGaLTXsCGAEwD2DePnRRu0IdEhgAviHlA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA

Repro Code

class TestObject {
  public value: number;
}

class Test {
  private testObj = new TestObject();
  private testObj2 = new TestObject();

  public test(): void {
    this.testObj.value = 10;
    // this.testObj2.value = 10;
  }
}

ESLint Config

module.exports = {
  "rules": { "@typescript-eslint/prefer-readonly": "error" }
}

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}

Expected Result

testObj and testObj2 is never reassigned; both should be marked as readonly

Actual Result

If we're setting a property on the testObj, eslint does not recognise it should be marked as readonly.
(Uncommenting line #7 should not make the eslint error go away.)

Additional Info

No response

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