I can't have this happen consistently. I have cloned and placed using structure blocks a lot of them, and only about four pairs had it happen. In the screenshot, for example, the fourth from the right is also triggered without an entity. Out of all the ones I know I have placed by hand, none has had the problem. That much I can say.
I updated the description. Sory for the lack of clarity. It's just I can't figure out what triggers this behaviour.
Maybe. I'd put the hoe's enchantability on par with the shears, not with other tools.