Mobs jump off 1x1 blocks into death

I realised this bug when I visited my pigmen farm (This design https://youtu.be/l9oteFEj770?t=1443)

The design is storing iron golems and zombie pigmen on 1x1 blocks which they didn't jump off. Now when I got close to my farm, many many blocks under it (Nether roof), I saw that the golems and the pigmen were jumping off of these blocks, something I don't think they should do, especially when they take fall damage.