I've got a very weird issue with my Ember application. I have a style.css file that is (unfortunately) 4030 lines long due to circumstances I cannot change at the moment. It is included in my app/index.html file like so
<link rel="stylesheet" href="assets/style.css">
So. After researching and trial and error, I've found that the reason this was a problem was because I named my main css file 'style.css', which is a very common name for a .css file. One of my vendor packages (still don't know which one) had a .css file also called style.css, and during compilation, the vendor .css and my app's .css clashed with each other.
Changed the name to 'myCompany_style.css' and everything works fine. Also as per @kumkanillam's suggestion, moved all css compilation to app.css
Will edit answer to include issue link if I can re-find it.