Repro

.eslintrc.json

{
  "parser": "@typescript-eslint/parser",
  "plugins": ["@typescript-eslint"],
  "parserOptions": {
    "ecmaVersion": 2018,
    "sourceType": "module",
    "project": "./tsconfig.json"
  },
  "rules": {
    "@typescript-eslint/prefer-nullish-coalescing": "error"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "rootDir": "src",
    "outDir": "lib",
    "strict": true
  },
  "include": [
    "src/**/*"
  ]
}

src/index.ts

declare const a: string | undefined;
declare const b: string;
declare const c: string;

const x = a || b || c;

Expected Result
The last line should be fixed like this:

const x = (a ?? b) || c;

Here parentheses are required because nullish coalescing operators and logical operators cannot be mixed.
Since logical operator || is left-associative, it is natural to add parentheses in left-to-right manner.

Actual Result
eslint --fix outputs this:

const x = a ?? b || c;

This is not a valid syntax and we get the following error.

$ /path/to/project/node_modules/.bin/tsc --build .
src/index.ts:5:16 - error TS5076: '||' and '??' operations cannot be mixed without parentheses.

5 const x = a ?? b || c;
                 ~~~~~~


Found 1 error.

Additional Info

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