I've been seeing this alot and remember seeing it before. I am in GRID in LR and select 20 raw files. I choose CMD+E to send them to Photoshop. Photoshop opens and the files start loading into the workspace. I go to do a Photomerge and I see the list of images in the workspace are incomplete. It's random. I checked the bottom of the list to see if they were sorted to the end. No, it's all sorted A->Z. This example was missing 4 selected images.