When I create a an eblast but don't want to send it right away I add all the info and the lists. But when I hit save in the new custom code editor and exit the eblast editor the lists I have attached aren't saved like they would be in the current code editor and I have to select them again. Also if you are selecting multiple lists there is no way to check which lists are selected without scrolling through the entire list of choices. I like the current one where you select the lists, save your choices and then it shows which lists are selected.
It appears this is a known issue, and our engineers are looking into it. I've assigned your case to them for further review. If and when they have an update, they'll make sure to notify you. In the meantime, the only workaround would be to re-select the lists, if you do make any future edits to a custom code email.