In those graphs, it appears as if the velocity values are set only once, on Event Begin Play. At that point in time, it would make sense for them to be 0,0,0 because nothing has happened yet. The variables in your pawn, Angular Velocity and Linear Velocity, are not reactive in the sense that they automatically update when something happens.
So, to fix this, your implementation of the interface function GetVelocity must make calls to GetPhysicsAngularVelocity and GetPhysicsLinearVelocity directly when the function is called, and return those (up to date) values.