Stryker for Typescript - mutations survive but fail in local tests

Question

I've been trying to run Stryker on my typescript repo for a while, and continue to get surviving mutants where I don't think they should be. I have manually added some of the mutants to my codebase, and my tests fail - some for compilation reasons, but generally because the code is properly under test.

NB - I'm working on mac, I know there have been similar questions on windows

Any ideas?

stryker.conf.json:

{
  "$schema": "./node_modules/@stryker-mutator/core/schema/stryker-schema.json",
  "_comment": "This config was generated using 'stryker init'. Please take a look at: https://stryker-mutator.io/docs/stryker-js/configuration/ for more information.",
  "checkers": ["typescript"],
  "coverageAnalysis": "perTest",
  "ignoreStatic": true,
  "logLevel": "info",
  "mutate": ["src/**/*.ts"],
  "packageManager": "npm",
  "reporters": ["html", "clear-text", "progress"],
  "thresholds": { "high": 80, "low": 60, "break": null },
  "testRunner": "jest",
  "tsconfigFile": "tsconfig.json",
  "typescriptChecker": {
    "prioritizePerformanceOverAccuracy": true
  }
}

jest.config.js:

/** @type {import('@ts-jest/dist/types').InitialOptionsTsJest} */
module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  testMatch: ['**/__tests__/**/*.ts', '**/?(*.)test.ts'],
  testPathIgnorePatterns: [
    'node_modules',
    '.cache',
    'public',
  ],
  collectCoverage: true,
  collectCoverageFrom: [
    'src/**/*.ts',
    '!src/**/*.d.ts',
    '!src/**/__utils__/*.ts',
  ],
  coverageThreshold: {
    global: {
      branches: 100,
      functions: 100,
      lines: 100,
      statements: 100,
    },
  },
  setupFiles: ['./.jest/setEnvVars.js'],
  silent: true,
  reporters: [
    'default',
    [
      'jest-junit',
      { outputDirectory: './test-results', outputName: 'jest.xml' },
    ],
  ],
};

tsconfig.json:

{
  "compilerOptions": {
    "lib": [
      "ESNext",
      "DOM"
    ],
    "module": "commonjs",
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "removeComments": true,
    "preserveConstEnums": true,
    "strict": true,
    "alwaysStrict": true,
    "strictPropertyInitialization": false,
    "strictNullChecks": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitAny": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "allowUnreachableCode": false,
    "noFallthroughCasesInSwitch": true,
    "target": "ESNext",
    "outDir": ".build",
    "declaration": true,
    "sourceMap": true,
    "baseUrl": ".",
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,
    "allowJs": false,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true
  },
  "exclude": ["./.build/**/*", "./node_modules/**/*"]
}

versions:

    "@stryker-mutator/core": "^7.1.1",
    "@stryker-mutator/jest-runner": "^7.1.1",
    "@stryker-mutator/typescript-checker": "^7.1.1",
    ...
    "ts-jest": "^27.0.4",
    "typescript": "^4.3.5"