https://youtu.be/BVUHNGSrhSo (really obvious @ 4s)

I'm not sure of the exact cause but my best guess is that it has to do with some of the changes made to the fov when rendering as it happens mainly with my sword and when my sprint (and subsequently fov) is changing.

Spectating a player in MC 1.8.9 / RM 2.6.1 doesn't show the held item correctly:

The item is located correctly, centred at the bottom of the screen or vanishes completely depending on how the yaw changes - regardless of pitch.
When changing the rotation direction, the behaviour doesn't flip immediately, but at 0° and ±180°.

Turning clockwise:
positive angle -> item is where it should be, in the bottom right corner
negative angle -> item appears centred

Turning counter-clockwise:
positive angle -> item vanishes
negative angle -> item is where it should be, in the bottom right corner