[Phillip Fischer]

Thank you for the confirmation that it theoretically should be working. I did have both visible in editor and render keyframed, but neglected to say as much.

After some further troubleshooting, it seems that it is caused, at least in part, by having the Motion Vector selected as a Multi-Pass rendering layer. Not sure why this is happening, much less what that layer does, but maybe this information will help others. If I find out more, I will update.