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=5.8.2&fileType=.tsx&code=MYewdgzgLgBFCm0DyAjAVgLhgJXqATgCYA80%2BAlmAOYA0MYArgLYrz4DaAugHwwC8MAN4BfAFChIsANbwAnvxgBGAAwBuUaITJ07GbM4wA-IYFd1mxFFRpdczgDoADgwgALABQqAlKqA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Y6RAM0WlqYSNkAC1pkA9gEMkyMswDm6KL2jjokADThseIqVlUa9ZIxasm42rCZNEZFMknRCY8UwAmlfJTdLIKtUhtAF8QYKA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3QAacDUhFYAVSLQ%2Bg9ABMAkkVXoAHmoCC0aOhLCwo9FIC%2BIK0A&tokens=false

Repro Code

const testObj: Record<string, number[]> = {}
const key = 10;

testObj[key] ??= [];

// the below code works fine for some reason which is why you also need 
// the rule @typescript-eslint/prefer-nullish-coalescing to be active for this bug to show
// if (!testObj[key]) {
//   testObj[key] = [];
// }

testObj[key].push(10);

ESLint Config

{
  "rules": {
    "@typescript-eslint/prefer-nullish-coalescing": "error",
    "@typescript-eslint/no-unnecessary-condition": "error"
  }
}

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": true,
    "noUncheckedIndexedAccess": true
  }
}

Expected Result

Shows an error showing that using a number for the key instead of a string is the issue.

Actual Result

Shows the following error

Unnecessary conditional, expected left-hand side of ?? operator to be possibly null or undefined

despite that not actually being the issue

Additional Info

No response