Cobblestone wall bug with spider

1. make a 10x1 cobblestone wall out of cobblestone wall block.
2. spawn a spider
3. hit the spider
4. fly over to other side of cobblestone wall
So i am playing on flat terrain and i have made a cobblestone wall all around my village. so i am walking and i see a spider outside my village and i hit it and it just somehow crawls over my cobblestone wall. i have no idea how. it just did. not sure if this is a bug or not, but just to be sure, i reported it.