Possibly due to how Luna recalculates rotations. For example, if a rotation is being applied to the Z axis in Unity, the X axis will also have its values changed as the rotation happens. Then on the next frame the X axis has a new value, the rotation get applied to the Z axis again, which in turn causes the X axis to snap back to its original rotation. This switching happens which frame between the two rotations and therefore flickering occurs.
In Unity, set the game object to the the correct rotation before runtime. For example, if a spinning crosshair image needs to lie flat on the a grass surface during gameplay, then ensure that the crosshair is also flat on the surface in the scene before runtime. You can then apply a the spinning rotation to a different axis.
Also try setting the rotation values to 0 in the Unity scene before runtime, and only rotating the one axis needed to turn.
Incorrect setup in scene (before runtime):
Correct setup in scene (before runtime):