Repro steps:
1. Create a universal lit / PBR graph
2. Create a new Enum keyword
3. Save the graph
4. Create a material with the shadergraph
5. Go back into the graph and rename the entries, and add an entry
6. Look at the material

Expected result: the enum will change
Actual result: not only will the enum not change, creating new materials is not a workaround. The only way to work around this that I've seen is to recreate the enum keyword.