Mask/Motion Blur bug and fix(?)

Hi! This is my first post to this forum, though I've used it many times as a resource. I ran into a strange issue and thought I'd report on it here, and see if anyone else has had it/come up with a better solution (I also reported this on the Adobe Forums)

I'm using After Effects CC 3.7 (updated today) so I dont believe there is a patch that solves this.

I have a mask on a precomp that looks like this when rendered correctly:





However, when I try and render the file as a lossless MOV through the render cue in AE ("Best Settings" and "Lossless") the mask shifts for a few random frames:



This does NOT happen if I render through Media Encoder (by "Add to Adobe Media Encoder Queue"

After a lot of trial and error, I found that this does NOT happen (ie the mask renders correctly) if I select "Off for All Layers" for Motion Blur in the Render Settings panel (circled here with the default setting):