Crabs having trouble walking off edges

My username is marisonicman91, and I was making a crab pen for my crabs when I started noticing that they had a hard time walking down one-block ledges. They twirled in place, and I couldn't even manage to push them off - I had to punch them. Attached below should is a video file demonstrating said bug. (If it isn't there, sorry. I'm reporting my first bug here.)