Site Builder Widget behaviour - Forums, Photos and Timeline
Installation and Configuration
What you probably want is not _updated, but forum_updated:
{jrCore_list module="jrForum" order_by="forum_updated desc" limit="5"}That appears to be the correct one here for me, it may be that I'm on newer code thats coming out with JR6 though, so not sure if it will give the same results for you.