Mobs on a lead occasionally can't ever enter a nether portal again

The bug

So far, I wasn't able to exactly find out when this bug happens. It first appeared when I first tried to push a cow on a lead into the nether portal and it didn't work. After that it took a couple of times before it happened again. As you put the lead off, the cow is still not able to enter the portal. The video below shows the issue.