It appears that the custom category broke because the parent category name changed in KSP version 1.3. It used to be "Filter by Function" (as seen in EditorCategory.cs) but is now "Filter by function" (note the lower case "f"). I'm not sure why it would have changed, since this is the name used internally, not the localized name. Fixing the name and rebuilding Infernal Robotics made the custom category work for me.