Channel value does not follow animation keyframes, depends on start frame?

I have two animation curves from Maya driving the x-rotation of a pair of planes.
Curve A runs 00---00->90---90->00---00.
Curve B runs 90---90->00---00->90---90. (6 keyframes each)
So when planeA is flat, planeB is upright.
During playback, PlaneA does not reach 90 degrees; it stops short somewhere around 80deg and also appears to lift slightly in y-translation. The planes have no other animation data other than x-rotation.
Keyframe Reduction is off. The animation curves are correct when viewed in Maya.

If I pause the animation during a broken phase, select the misaligned plane, manually enter '90' in the channel box, and resume, then it will behave until it is stopped.
Is that normal? Shouldn't a manual adjustment that corrects the 90 position throw off the 0 position? (It doesn't)

That's not the weird part...
If I set the prefab to play the animation from the halfway point, then PlaneA is fine, and PlaneB won't reach 90 degrees and translates slightly skyward.
So if the plane is upright at the beginning, then it is fine. If it's flat at the beginning, then it is broken when upright.
Can anybody explain that?

 

Nothing, eh?