The green "in-game" border was blocking raycasts to user card UI elements, which I hadn't noticed until now since the Vivox volume controls are the only interactable thing therein.

Assets/Prefabs/UI/LobbyUserList.prefab

   m_EditorClassIdentifier: 
   m_Material: {fileID: 0}
   m_Color: {r: 0.14320946, g: 1, b: 0, a: 0.392}
-  m_RaycastTarget: 1
+  m_RaycastTarget: 0
   m_RaycastPadding: {x: 0, y: 0, z: 0, w: 0}
   m_Maskable: 1
   m_OnCullStateChanged: