Productivity Analytics: Trendline chart not loading because 'n.response' undefined

Summary

Similar to Issue #219933 (closed) - A customer has been experiencing an issue with viewing productivity analytics trendlines, only they are not getting TypeError: Cannot read property 'status' of undefined - they are repeatedly getting:

Uncaught (in promise) TypeError: n.response is undefined
    Ne https://on-premise-gitlab-host/assets/webpack/commons-pages.analytics.productivity_analytics-pages.groups.analytics.productivity_analytics.9d853785.chunk.js:1
    m https://on-premise-gitlab-host/assets/webpack/main.6d2ca3af.chunk.js:133
    dispatch https://on-premise-gitlab-host/assets/webpack/main.6d2ca3af.chunk.js:139
    dispatch https://on-premise-gitlab-host/assets/webpack/main.6d2ca3af.chunk.js:133
    dispatch https://on-premise-gitlab-host/assets/webpack/main.6d2ca3af.chunk.js:133
    Ge https://on-premise-gitlab-host/assets/webpack/commons-pages.analytics.productivity_analytics-pages.groups.analytics.productivity_analytics.9d853785.chunk.js:1
    promise callback*Ge https://on-premise-gitlab-host/assets/webpack/commons-pages.analytics.productivity_analytics-pages.groups.analytics.productivity_analytics.9d853785.chunk.js:1

The same manifestation - where they click back-and-forth between merged_before (From: ) dates in the UI results in the spinning Trendline.

GitLab logs from the server, showed one single unexpected 0 return code in Rails, and a 302 in Workhorse, which looked like the group and merged_after/merged_before params were not included.

However all other requests to /groups/apps/-/analytics/productivity_analytics, even with n.Response errors result in 200 responses back to the browser, and what looks to be appropriate JSON data.

At the time of the report, customer was on GitLab 13.10.2 Self-managed.

Steps to reproduce

I was unable to reproduce this. I could only generate status errors.

In code investigation, I took the data the customer included in the request responses and hand parsed that with the transformScatterData function (creating/defining supporting console functions). The data parsing and output seemed fine.

The only place I saw a response get involved is in the fetchChartData: ( v13.10.2-ee code here ) - coming out of the call to axios.get - but I don't see any condition from reading the axios docs that should have resulted in the response being undefined.

What is the current bug behavior?

Trendline doesn't display, error in Browser console.

What is the expected correct behavior?

Trendline displays, no errors.

Relevant logs and/or screenshots

Please see the referenced ticket link for an .mp4 demonstrating the error behavior.