Hello and welcome aboard!
My fix for my problem was in the Render Properties → Motion Blur → Shutter field (default is set to .5) adjust that value to 0.1. Then I set the start frame to 41, and end frame to 42, then rendered the animation. The 2 problem frames were rendered correctly and saved over the problem renders.
Here’s the deal on that. When an object has a child of constraint on it, it appears to move with the object the constraint targets. Thing is, it doesn’t actually move, it appears to move because it’s constrained to move. So in my case, frame 41, the helmet is constrained to the hand, on frame 42, the helmet is constrained to head. In between frame 41 & 42, both constraints influences are at .5 and that’s why you see the movement.
I read this thread the other day and @carterbk explains how motion blur works, and how to work around it.
So in my case, at frame 41, the helmet should be constrained to the hand, by frame 41.5, the helmet should be constrained to the head. So the switching of constraints happens before the frame range the motion blur looks at.
At the end of the day, it was easier just to render the 2 frames with reduced motion blur instead of adjusting keys spacing on subframes.
Hope this made sense!
Randy