This appears to only be an issue in Mobile Preview. Tests on a mobile device showed touch to work correctly.
If the user presses down and drags off screen without lifting their finger/releasing mouse button then the "Get Input Touch State" node does not return false (for the Is Currently pressed pin). It continues to read as true until the screen is touched/clicked again
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-13210 in the post.