You can't damage entities with a high enough strength level.

As you can see from the attached clips, if you place a boat while with strength, then attempt to break it, then the boat won't be breakable. I could not identify the level it starts, but the level I have it set to is: 255 and creative or survival. You can break it with commands such as /kill @e, however.

Steps to reproduce: 1. Place a mob egg, painting, armor stand, etc, or find one that has spawned naturally.

2. Try to hit it.

 

Observed results: you can't damage/break the entity. You can kill it with commands such as /kill @e.

Expected results: you can hit the entity so that it gets killed/damaged.