I'm using AEM6.4.4 and have created a user group and added couple of users in it. The issue is, when I double click on the user group, I'm able to see the user A that I have just added to the group but when I search for the user A and double click on it after it appears in my search result, and open the 'Groups', I can't see the user group.
I remember this sometimes happens with classic UI console. Could you please try the same with touch ui console. Plus with classic UI it wont show any proper error if and all any exception happened but the same is not case with touch ui.