Based on the minecraft wiki, a hopper that was locked does not push/pull items in the same redstone tick of when it was unlocked. It is expected to have a 1 redstone tick delay.

But it seems that it is processed in the same tick, instead of observing the 1tick delay.

Reproduction : 

In this setup, when the torch on the target block turns on, it will power the block below the dropper, and trigger it. It will also unpower the redstone torch that is locking the hopper.

When processing the events tick by tick, we can see that the tick in which the redstone torch turns off, the hopper already has moved the item in the dropper.
Since the dropper has a 2redstone tick delay, it will not have time to eject it's item before the hopper pushes into it, because the hopper is ignoring the "unlocked" delay. (the expected behavior is: 1 tick for torch to turn off + 1 tick for hopper to be unlocked == dropper delay).

You can also easily see it by placing a full stack in the dropper, and the same item in the hopper. The behavior makes the items in the dropper always be 2 stacks, 1 of 63 and 1 of 1, instead of staying a full stack of 64 (if the dropper was fireing before the hopper inserts).