In Resolve 16.2.3 on macOS 10.14.6, the "Undo" function does not work with any operations performed on a Shared Node.
When you switch to a non-shared node in the same node-graph, then changing a parameter in that node, the Undo function works as expected.
An interesting observation is that when you initially perform the Undo (either via keyboard shortcut, or from the "Edit/Undo" menu command), the node thumbnail (and
only the thumbnail....not the actual Viewer )momentarily flashes to display the results of the previous operation that you want to undo to, but then the thumbnail reverts back to the current state of the grade.
Logfiles attached. Also, here's a screencast of what I'm seeing on my system:
Resolve Studio 18.0.2 / Decklink Mini Monitor / 14" 2021 Macbook Pro Max (macOS 12.5.1, M1 Max) / 32GB RAM