Refused to apply style from <url>.css because its MIME type

Describe your question in as much detail as possible:
I have some users (not all!) who are failing to render the Gitlab webpage for our on-prem install of 12.10. We recently upgraded from 12.6 going through all of the necessary upgrades in order.

What are you seeing, and how does it differ from what you expect to see?
The users expect to see the normal Gitlab login page, but they get the page without any proper CSS.

Consider including screenshots, error messages, and/or other helpful visuals
The error when running Chrome analytics via F12 is:
Refused to apply style from ‘http://server.com/assets/application-ed4780bb05c30e3c145419d06ad0ab3f48bd3004a90fb99601f40c5b6e1d90fd.css’ because its MIME type (‘text/plain’) is not a supported stylesheet MIME type, and strict MIME checking is enabled.

Refused to apply style from ‘http://server.com/assets/highlight/themes/white-3144068cf4f603d290f553b653926358ddcd02493b9728f62417682657fc58c0.css’ because its MIME type (‘text/plain’) is not a supported stylesheet MIME type, and strict MIME checking is enabled.

What version are you on (Hint: /help) ? and are you using self-managed or gitlab.com?
12.10 EE

What troubleshooting steps have you already taken? Can you link to any docs or other resources so we know where you have been?
All of the troubleshooting steps seem to suggest modifying the server files, however this makes little sense because it is only happening to some users, not all.