Repro
Using the repo code from https://github.com/WGroenestein/typescript-eslint-prefer-readonly-class-mixin

1. run "npm install"
2. run "npm run build"
3. see that the build compiles without errors
4. run "npm run lint"
5. see that it fails with error

file.ts
  7:3  error  Member '_name' is never reassigned; mark it as `readonly`  @typescript-eslint/prefer-readonly

✖ 1 problem (1 error, 0 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

6. run "npm run fix-lint"
7. see that it prefixed "_name" in "file.ts" with readonly
8. run "npm run build"
9. see that the build fails with error

file.ts(10,9): error TS2540: Cannot assign to '_name' because it is a read-only property.

Expected Result

Fields which are assigned outside the constructor (e.g. setters) in mixin classes are not marked as not assigned / readonly

Actual Result

They are reported as errors, and when running --fix it breaks the build

Additional Info

The debug log can be found in the repo (https://github.com/WGroenestein/typescript-eslint-prefer-readonly-class-mixin/blob/master/debug.log)

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