Currently, there is an option on threaded discussions for instructors to enable the 'liking' of a reply/post. There is an additional granular setting that let's only someone with a grader role like the posts. However, if you turn a threaded discussion into a group discussion then this all changes. Users in a group are granted escalated permissions as groups are independent of courses and there is no option to grade in a group page which eliminates the Graders role. Wait, What?! Yes, this intended behavior. So the options still exist to allow liking, BUT if you have selected the additional option 'only graders can like' then not even the instructor can like a reply. Why is this option even there? Because enough people haven't noticed to let Instructure know. Why wouldn't an instructor who set up groups be allowed to be the only one to show a presence and like a group discussion? Join me in voting this idea to their attention.
For those instructors who have found this post and agree that this is a bug and not a feature, your workaround is to turn off the option for 'only graders can like' on group discussions. While this would permit students to like each others posts, it would allow you to do the same as well.