Yeah its a 

Bug

Thats's the way it works. Take some time to activate after hit. AfaIk the time correspond to distance

Actually, Hemlock has a short delay before applying Daze effect, but after Daze activates, there's a 1s time which most players use to move quickly to cover. And, unlike Orion. Hemlock's ECM Shot is difficult to fire, and has a high chance of missing it's target at long range. I use Hemlock mostly to get rid of Lancer campers, but those players often abuse the 1s delay (after Daze has been applied). In the worst cases, their teammates kill my Hemlock before I use ECM shot again on the Lancer or other player who tries to attack Hemlock. And once the Lancer camper knows when there's a Hemlock player (usually when they get affected by Daze), then somehow the entire opponent team tries to eliminate my Hemlock. And the 1s delay (due to the bug) helps the Lancer camper to perfectly escape (in some maps) and would spam the game with Savants, Geminis, Tetras and Continuums

Guys as I said, the mech is moving after the Daze is applied, they have a solid purple icon over them and still move. This shouldn't be possible. 

Look at your ping next time this happens. Typically if you have high ping there'll be an extra delay