Has anyone else had issues trying to sort fields on related tables? All of my other layers can be sorted. I thought it might be an issue that was brought up earlier with pagination with our Oracle db so I moved it to a current version of Postgresql and had no luck there either. I've also tried multiple themes to eliminate that possibility. This is Web AppBuilder 1.3 Developer with 10.3.1 Server.
I can confirm that sorting related tables by column does not work in the Attribute Table widget. You need to contact esri tech support and report this bug.