There is a rotation field in custom geometry, which has three floats: yaw, pitch and roll. However, only roll rotation works for player. Yaw and pitch do not affect rotation at all. Below is how it looks like:

And here is what I expected it to be:

How to reproduce?
Create player with custom geometry like that one, don't know if it's possible to do in singleplayer.

Why is it important?

With this bug server creators can't rotate custom models how they want to and can't rotate some parts at all(arms). So with that bug fixed custom models will become even cooler and, well, rotated!