ry and rym are always -90 with mobs!

Whenever I use ry and rym on zombies, it acts like the zombie is pointing at -90 all the time, even if I can clearly see it facing other directions. I haven't tested with other mobs, but I am pretty sure it doesn't work with them as well. I really hope this glitch gets fixed soon, because I need to know a zombie's rotation to make the redstoning for my stealth-themed adventure map.