When editing an existing curves layer... Make any change to the curve and then Command Z to undo. The undo works and the history panel shows a step back. All good. Now Shift Command Z to redo. The history panel shows the step forward but the curve remains in its current state. The undo state. There is now no way to redo the change. If the first undo is done using the history panel, not Command Z, everything works as expected. It's only the key command that breaks it.