In 4.26.0 you no longer can change any scalar/vector values in PP material graph (NOT in instance) in realtime. You need to recompile entire shader again to see changes.
In 4.20 and also oldes version it works as expected and changing values of already compiled shader change everything what you see in viewport.
Why it was changed in 4.26?
I know about material instances, no need to talk about it.