Yes Knives can and do become dull and lose their sharpness in the dishwasher so it's not recommended to place knives in the dishwasher.
I've placed knives in the dishwasher and they did become dull because of the dishwasher machine detergent being abrasive.
The abrasiveness of the dishwasher detergent takes the sharpness edge right off the knife so if you do put knives in the dishwasher you'll have to resharpen the knives to be able to properly use them.
Knives should just be washed by hand and not placed in the dishwasher because every knife I put in the dishwasher to clean they all become dull and needed to be sharpened again.
After experiencing the knives becoming dull in the dishwasher I now just wash them by hand.