Jest coverage shows different results when running single test file vs all tests

I'm experiencing frustrating Jest coverage inconsistencies in my React project. When I run tests for individual files in isolation, I get perfect 100% coverage as expected. However, when I run the complete test yarn test --coverage , the coverage results become unreliable and inconsistent. \certain lines randomly show as uncovered across different test runs, even though I know these lines are being properly tested.

The problem seems to affect some files particularly, and they "take turns" showing incomplete coverage. One run might show file A with missing coverage, while the next run shows file B with missing coverage, seemingly at random.

jest.config.ts

import type { Config } from 'jest'; import nextJest from 'next/jest.js'; const createJestConfig = nextJest({ dir: './', }); const config: Config = { coverageProvider: 'v8', testEnvironment: 'jsdom', clearMocks: true, restoreMocks: true, resetMocks: true, preset: 'ts-jest', testEnvironmentOptions: { url: 'http://localhost' }, setupFilesAfterEnv: ['./jest.setup.tsx'], collectCoverageFrom: ['src/**/*.{ts,tsx}'], coveragePathIgnorePatterns: [ '/node_modules/', 'src/app/layout.tsx', 'src/i18n/', 'typings.d.ts', 'stories.tsx', 'sanityClient.ts', 'src/app/stores/index', ], moduleNameMapper: { 'lucide-react': require.resolve('lucide-react'), '^components/(.*)$': '<rootDir>/src/app/components/$1', '^stores$': '<rootDir>/src/app/stores', '^stores/(.*)$': '<rootDir>/src/app/stores/$1', '^designSystem/(.*)$': '<rootDir>/src/app/designSystem/$1', '^lib/(.*)$': '<rootDir>/src/app/lib/$1', '^pdp/(.*)$': '<rootDir>/src/app/pdp/$1', '^akills/(.*)$': '<rootDir>/src/app/skills/$1', '^home/(.*)$': '<rootDir>/src/app/home/$1', '^@/(.*)$': '<rootDir>/src/app/$1', }, }; export default createJestConfig(config); 

jest.setup.tsx

import 'react'; import '@testing-library/jest-dom'; export const mockEmblaApi = { scrollPrev: jest.fn(), scrollNext: jest.fn(), scrollTo: jest.fn(), selectedScrollSnap: jest.fn().mockReturnValue(0), canScrollPrev: jest.fn().mockReturnValue(false), canScrollNext: jest.fn().mockReturnValue(false), on: jest.fn(), off: jest.fn(), reInit: jest.fn(), }; export const updateMockEmblaApi = (updates: Partial<typeof mockEmblaApi>) => { Object.assign(mockEmblaApi, updates); }; jest.mock('embla-carousel-react', () => { return { __esModule: true, default: () => [jest.requireActual('react').createRef(), mockEmblaApi], }; }); module.exports = { updateMockEmblaApi, mockEmblaApi }; const mockSanity = { fetch: jest.fn().mockResolvedValue([]), }; jest.mock('./src/app/lib/sanityClient', () => ({ sanityClient: mockSanity })); jest.mock('./src/app/stores', () => ({ useCategoriesStore: jest.fn(), useSkillsStore: jest.fn(), useFiltersStore: jest.fn(), useSortStore: jest.fn(), useRefinementStore: jest.fn(), useSkillProductsStore: jest.fn(), usePortfolioSettingsStore: jest.fn(), })); jest.mock('next/navigation', () => ({ ...jest.requireActual('next-router-mock/navigation'), notFound: jest.fn(), })); 

What I've already tried:

• Added resetMocks: true to Jest config (but this introduced React hook errors)

• Used clearMocks and various reset configurations

• Attempted to isolate modules

• Fixed numerous individual tests

I'd like to diagnose and fix this issue. any help is appreciated.