CSP issue with iframe loaded from chrome extension - google-chrome-extension

Question

I'm developing a chrome extension that renders and sidebar iframe in Gmail.
A while back, we ran into a Content Security Policy issue that broke our extension and ran into a well-known error that shows up in your console as
Refused to frame '[OUR DOMAIN]' because it violates the following Content Security Policy directive: ...
To fix it, we had to rewrite the CSP header as explained in this blog post.
However, recently, we've been getting the same error, even after having implemented the fix suggested in the blog post.
As you can see, even though we added the chrome.webRequest.onHeadersReceived listener to overwrite the CSP header, our domain doesn't seem to be working. As a result, our sidebar does not load.
Notes and attempts:
Note that upon refreshing the entire page, the sidebar will sometimes load successfully %50 of the times, which leads me to believe that we're not adding the above event listener fast enough. To solve this, I pulled the event listener out of our background class and placed it at the top of the background js script. However, this issue seems to still persist.
We've tried to remove and re-add the extension but that didn't yield any permanent results.
Throughout testing, I also noticed that adding another extension that implements a similar override on the CSP header will cause our extension to break. Removing and re-adding our extension brings us back to the 50-50 issue in the first note.
[Update]
It seems that being able to reproduce it %50 of the times has to do with the version of chrome. After upgrading it to 72, I'm now able to reproduce the issue. However, doing a hard refresh on the page will sometimes allow the page to load.
More details on our setup: What we do is basically have a background.html which contains html templates and we have one template that contains an iframe tag. To get the template with the iframe, we do a sendMessage, grab the response, which is the template in string, convert it to DOM element via jquery, append it to body, and change the iframe's src attribute. What happens is the iframe loads fine initially, but when you click on any link in the iframe that would redirect to another page, the same error above shows up.
I can provide snippets and more info if needed.