For example you have a property group @myGroup which can contain any css property like color, bg, etc. It can be created in ACP. In this property-group you have a filed called "Miscellaneous". If you include there a simple multi-line property it works as it should. But when you open a css template which contains this property-group (and multi-line property inside it) and save it - XenForo removes this simple property from property group.