This is a redstone travel bug
Diagram here
B= block C=Comparator D=Dispenser(Full) R=Redstone X= Air

R B B
B C D

The comparator when powered by the dispenser sends power into block below redstone powering dust on top if left that way no matter what you do the container should it be a dispenser/dropper can NEVER fire however breaking the redstone and replacing it doesnt cause the dispenser to fire

Removing the redstone allows the dispenser to fire as well as setting it up like this
R X B
B C D

There is no path for the redstone to use to get to the dispenser in sample 1 AND if there was the dispenser should FIRE not sit like a lazy duck