If a chicken tries to jump on top of a block with another block over it, the chicken will take damage.
The chicken must jump into the space, it can walk through a hole with no issues.

In the example picture, the red arrow points to where the chicken jumps

I have recreated this bug with multiple block types as well as other 1 block high mobs. As far as I know, any block type works and the only mob affected is the chicken. The bug also persists with any blocks surrounding the 2 blocks.