When your Jest tests used to pass but suddenly started to fail, this may be related to cache.
In my case, the test coverage was reporting uncovered lines. When I cleared my Jest cache, the tests passed again.
To clear cache with Yarn:
yarn jest --clearCache
yarn run v1.9.4 ./node_modules/.bin/jest --clearCache Cleared /var/folders/c8/hg7gtddj49qgpvghtv2grkp9j21wxz/T/jest_r86htr ✨ Done in 0.54s.
Or with npx:
npx jest --clearCache