This has been found when trying to fix https://issues.liferay.com/browse/LPS-57271.
The problem is that "ant build-css" generates a ".sass-cache" directory in the chat-portlet.war file, beside the main.scss file. Unfortunately, that directory is not read any more by DynamicCSSUtil because the code has been removed and the CSS is never served.
Test case: deploying any legacy WAR portlet and asserting that its CSS styles are not broken will do. For example, you can check if the fix works by checking