Mobs running in circles and not jumping when chasing/running away

All mobs when they are going to go down a block ( fall 1 block high to the next block ) they just begin twisting and spinning in circles.I also noticed that they didn't jump to climb blocks either, i put a chicken on a 2 x 2 square and punched her, she began to run inside of this 2 x 2 area. One more thing to notice is that if you save and exit of a world , and then come back, mobs will be already spinning , not being needed to climb - fall blocks, (superflat worlds can prove that) but after - before that bug, they will only spin on the previous conditions.