Not sure if I should file a new ticket, since this one is 11 months old, but I'm having the same issue.

You can see that alpha isn't correctly shown in the node previews, although the final preview is correct. Not a showstopper, but definitely unhelpful when experimenting...

(The top "Add" node is plugged into Emission, and the bottom "Max" node into Opacity.)

Sorry, I'm still learning this, and the fact that I once was able to plug a Vector3 in opacity clip (as seen in the left part of the screenshot) confused me...