The Get Physics Linear Velocity node seems to only output a vector of 0.0, 0.0, 0.0 no matter what the velocity of the object it's measuring is.
Tested in //UE4/Release-4.27 @ CL 17735300 and this did NOT occur, so marking as a Regression
A value of zero is printed out on the viewport
The value printed would be representative of the ThirdPersonCharacter's velocity
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-135948 in the post.