Guest users in unable to see DDL form even when the portlet, data list and data definition all have guest view permission assigned. According to
LPS-27430 this was already fixed in 6.1 CE GA2 but it's now broken in 6.1 CE GA3 as well as 6.2.x and it doesn't even work in 6.1 CE GA2 so maybe it was never fixed correctly. The error you'll get when viewing a page with the portlet is "You do not have the required permissions." instead of seeing the form.