Leashes of the leashed mobs are gone after several hours

I used my wolf/dog in my minecraft world as a tie of a well. And after making other projects for hours, the leashes are gone. I didn't make screenshot of the gone leashes, because I remade the well. And there's another one, If you leave your leashed horse(s) for a long time like mining, etc, the leash(es) will disappear.