How to reproduce

1. Run:

   /setblock ~ ~ ~ spawner{SpawnData:{id:"minecraft:leash_knot"}}
   

2. F3+B
3. Go near the spawner or farther away from it

Result: When your head is inside the spawner (gamemode 3), the hitbox is offset by 0,5 (the offset direction rotates with the displayed entity). When you pass the mark of being exactly one block away from the middle of the spawner (head position), it jumps to a 0,5 higher offset and that for every block distance mark.

Code analysis

Code analysis by marcono1234 can be found in this comment.