Previewing nodes in the Material Editor is still broken (has been since 4.8 P1). Create a node in the editor, right-click and preview it. The node will turn blue but the Previewing text won’t show up. Now stop previewing that node, the previewing text will show up and stay there until you create a new node and force the graph to refresh.
Easy to reproduce and I have an answerhub report here: [Repro] [4.8] Material Graph Doesn't Refresh Inputs Until A New Node SI - Asset Creation - Epic Developer Community Forums