Browsers cache web application despite of headers that disable caching

We regularly deploy an Angular based web application. Simplified this application consists of an index.html and multipe .css and .js files. Nginx is configured to not cache the / (equates to index.html) but cache the resource files css and js forever. This is feasible because the resource file names change for every update:

index.html -> include `my-timestamp.css`
index-html -> include `my1-timestamp.js`

The strange thing is that suddenly (it worked before for months) our main browser (Internet Exploder 11) uses an outdated index.html and tries to retrieve for example my-previous_version_timestamp.css. To make this even worse: the css can not be retrieved from the cache, some of the javascript resources fail too. If this would happen only with IE I would blame that one but the same happens with a recent Chrome too.

Headers for the root:

cache-control: no-cache
date: Fri, 24 May 2019 20:17:00 GMT
etag: "1dc09d84-322"
expires: Fri, 24 May 2019 20:16:59 GMT
last-modified: Sat, 26 Oct 1985 08:15:00 GMT

(others excluded)

Headers for the resources:

cache-control: max-age=2592000
cache-control: public
etag: "1dc09d84-12d68"
expires: Sun, 23 Jun 2019 20:17:02 GMT
last-modified: Sat, 26 Oct 1985 08:15:00 GMT

There is no proxy between the browsers and the webserver. We noticed one thing: it looks like our build process that builds the Angular application no longer creates "sane" timestamps. All files are from 1985 (you can see this in the last-modified header), before they carried the date they were built on. But I don't see how this could change the browsers behaviour.