Zombies can not pathfing on rail/slab combination.
How to recreate:
1.Make a row of slabs(10 slabs.)
2.Put any type of rails on both sides.(10 rails on both side.)
3.Summon some zombies one 1 side and villager on the other.
4.Watch how zombies don't walk over slabs/rails but go around them.
5.Fix the bug.