Wow... figured it out. It often just takes formalizing my issue to narrow it down. I did not have any motion blur on (I read somewhere that mr calculates 0.2 of a frame in either direction). But I forgot that I did have a motion vector 3D pass associated with the layer, and so of course that was it.
That's got to be a bug that it renders out the fractional frame when you have a mv pass. That's terrible.