Check if any of your combined CSS files have used "import" method. This can cause design breaking as imported CSS will not work when you combine them. For example:
Instead of using import, you can create one separate CSS file and write down all the imported CSS files code in that separate CSS file.
Or, you can include the file using "link" HTML tag, like this:
<link rel="style.css" href="style.css" type="text/css">
In Magento, you can include CSS files from