Hi,
If you are using SG optimizer by any chance, it has a bug, where it picks up some CSS files, but then it does not add it back in during the combine method. We already addressed that in the next release with a hacky workaround. I hope they will fix that soon, I see a lot of bypasses in other plugin codes as well just to get around this SG optimizer issue.
Until then, you can resolve it by changing this option: https://i.imgur.com/46DQfwq.png
