yes, presoak needed
yes, and it can kill them! What is likely happening, is the food is dry and as it soaks up water it swells. If the fish has it in it's throat or in its stomach when this happens it can plug up, particularly if it swells in the throat. I killed a danio this way, large pond sized food, too dry, greedy fish that swallowed it as soon as it hit the water.
If the food is dry and the fish are greedy, presoak the food. Just put it in a cup of tank water for a minute or 3 before you pour it in the tank.