,
I had this same warning appear for myself as well. Remove the intermediate and saved folders under your project folder. This should clear the invalidating cook warning. The only thing I have noticed though is that it will fix the warning for only so long. In my project it will come back every now and then but the solution is to remove those two folders. Sometimes removing the hidden UnrealEngine folder under your AppData folder will also clear any strange issues.
See my question about this here…
hope this helps.