It is related to MC-6785
When you activate a dispenser, any hopper attached is stopped.
In MC-6785 it is stated that if you want it to work (ie dispenser firing while hopper filling it), you'll have just to power the dispenser through a block which doesn't work (second screenshot)

What I expected to happen was...:
Having the dispenser firing anything I put inside teh connected hopper

What actually happened was...:
The hopper stopped as soon as the dispenser is powered

Steps to Reproduce:
1. put a dispenser and a dropper attached to it
2. power the dispenser (any ways)
3. try to put things inside the hopper and see that they are not grabbed by the dispenser as soon as the dipsenser is powered