My custom list style have a disc before the bullet because a property is specified in this css :
/cms/plugins/core-ui/resources/css/Ametys/form/RichTextConfiguration-Inline.css

/* -------------
       LISTS
   ------------- */
.mceContentBody ul,
.mceContentBody ul.disc
{
	list-style-type: disc;
	list-style-image: none;
}