If I select multiple photos in search results in All Photos, the "Group into Stack" operation is disabled. Why? In general, I'm having a hard time understanding when I can and can't group/ungroup stacks, and why. I haven't been able to find comprehensive documentation for this feature.
Stacks are designed around a per Album/Folder use-basis. Photos will allow stacking and the menu command will be active if the photos are in the same album or folder.
In the case of the All Photos special system collection, there are a couple of gotchas:
1. If filtering is active, stacking should be disabled. 2. If you happen to accidentally select two images that are in the same folder/album stacking can be allowed. If a third photo not in the same album or folder is selected, stacking will be disallowed.