It is absurd that you can't add attributes to all elements in a stack without expanding the stack (thereby defeating the purpose). Has anyone found a workaround for this?
So what is the purpose?
Why are you stacking?
Should you be using a collection?
A workaround: If you shift-click on the stack badge in the upper-left corner of a stack, it will expand with all its pics selected. Then as you add metadata, it will be added to all of them.
You can use the Syncomatic plugin to copy metadata and/or develop settings from the top of stacks to the other photos in the stacks.
Others have asked for an option controlling this behavior. Please add your vote and opinion to this topic in the official Adobe feedback forum: Lightroom: When selecting a collapsed stack, provide option to apply changes to all photos in the stack