All "Key" related BlueprintPure nodes cause the Blueprint to become dirty when the Blueprint is reopened.
Key Get Display Name
Key Is Float Axis
Key Is Gamepad Key
Key Is Keyboard Key
Key Is Modifier Key
Key Is Mouse Button
Key Is Vector Axis
Per James B.:
Just having any of these nodes in the event graph (not even hooked up) causes the blueprint to be flagged as "Dirty; needs to be recompiled." Tracking this issue down to the code shows that these nodes all call the function "FKey::ConditionalLookupKeyDetails()," which modifies a member property. This issue occurs in the UE4-Fortnite branch.
Blueprint is dirty once it's opened.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-22761 in the post.