This bug has been in the game since Anet started messing around with the Steal code around 2 months ago.
Essentially if you’re targeting something and your starting position is outside of Steal’s range, you cannot use the skill right after a teleport(i.e., Shadow Step, Infil Signet) even if that teleport lands inside Steal’s maximum range. The skill works just fine if you activate it a bit later after casting your teleport.
To make this easier to understand you can replicate this bug by going into the Heart of The Mists. Find a practice Golem, target it and stand right in front of it. Start walking backwards and away from it until you’re outside steal’s range( A red bar will appear under Steal when this happens). Once you’re there, use your Shadow Step to teleport on top of the Golem, immediately followed by Steal and you will see the message “Out of Range” pop up. If you repeat this process but cast Steal a bit later after the teleport, it will activate like it should.