New version of yarn package breaks React websocket (updated - cause found)

Question

This has got me absolutely beat.

I have a .NET Core 3.1 React SPA app which references some (private) scoped packages.

This morning I (published via GitHub Actions) a new version of one of the packages.

This new version, and any subsequent versions, have completely bricked my web site - sockjs-node and calls to fonts.google.com now are stuck in "Pending" and the whole thing hangs as shown here.

Normally the startup web traffic looks like this.

Downgrading to a prior version removes the issue completely - so I am assuming at this point it is package-related, trouble is:

• A source code diff doesn't show any differences in the package output (javascript library) other than the code change

• I reverted the code change and re-published, this didn't remove the error

• I completely nuked any cached browser entries, this didn't make a difference

• The setup-node action in GitHub Actions has recently been updated so I modified the workflow YML on my side to target the specific version prior (this also did not fix the issue):

  steps:
    - uses: actions/checkout@v2
    - name: Setup Node.js
      uses: actions/setup-node@aa759c6c94d3800c55b8601f21ba4b2371704cb7
  

• I thought it was possibly app-specific, so I referenced the new version on a separate app / codebase, this didn't make a difference (it also froze)

• Tested on Edge, Chrome, Firefox, all the same result: calls to websocket-related endpoints are stuck in pending

• Tried 'salting the earth' by deleting the local code and cloning the repo and re-installing node_modules - this didn't make a difference

• The actual changed code was to add the formik.isValid flag (from FormIk v2.1.2) to my returned React Hook (typescript) handler object:

  return {
      errors: formik.errors,
      handleBlur: formik.handleBlur,
      handleChange: formik.handleChange,
      handleRefresh,
      handleReset,
      handleSubmit: formik.handleSubmit,
      isDirty: formik.dirty,
      isLoading: state.isLoading,
      isValid: formik.isValid,
      record: formik.values,
      setFieldValue: formik.setFieldValue,
      setValues: formik.setValues,
      submitForm: formik.submitForm
  };
  

Typically if this has broken dependencies or something I'd have expected a build error, not this particular scenario. And again, after undoing this code change, the new packages still caused the issue.

Any help at all would be appreciated.

UPDATED

I found the cause of the issue - this problematic library was also a dependency of another library which was also running in the app(s); the new version of the built package was not propagating to the dependent. Once this was done, everything worked.