Thanks for the report - it appears this is a bug with 4.14 and they have fixed it for 4.15:
https://answers.unrealengine.com/questions/526492/new-ease-function-node-not-pure.html
https://answers.unrealengine.com/questions/524485/getting-a-warning-with-the-ease-node-saying-it-nee.html
But apparently they’re only warnings and shouldn’t harm anything, just a bit of a pain having the warnings there.