While applying object mask in Premiere Pro Beta Render fails

The project is 3D Vr180 8K settings. I separated the right and left images and placed them on top of each other in the timeline. I distinguished the clips by cropping the right on one clip and the left on the other. Then I used the object selection to outline my subject. I rendered the clip forward to it's completion and repeated the process on the left clip.

When I tried to export the clips together, Media Encoder showed a warning that certain clips were offline. When I examined the exported file, the mask was ignored and exported without the desired effect.