When you try to summon a goat with a rotation value, it just kinda nudges itself in one direction or the other, depending on the rotation value. Using the /data command shows that the goat does indeed have a rotation of the specified value, but it doesn't show it for some reason.

How to replicate:

1. summon a goat with rotation. Use /summon goat ~ ~ ~

What is expected: The goat is facing 90º
What actually happens: The goat only is nudged to its right a bit, but is rotated 90º

I tested other mobs like the axolotl, they work fine. It's just the goat that has issues.