1. Select a folder containing about a dozen photos, all *not* part of a stack 2. Group photos so that three or four stacks are created and all photos are in a stack 3. Expand all stacks 4. Select all photos 5. Group into stack (now a single stack containing all photos of folder is there) 6. UNDO
Expected result at step 6: Stacking goes back to the previous three or four stacks
Actual result: Apparently only one of the stacks is restored, and all other photos are now contained in "single-photo-stack" displaying the "photo is part of the stack" badge with the number zero [!] in it. Expaning/collapsing those "stacks" seems to hide/show the previously existing stacks, but they are definitely gone. The "unstack" command is not available to "break up" the "single-photo-stacks". A restart of LR does not help.
However, there seems to be a workaround: When selecting the "single-photo-stacks" together with a "normal stack", the "unstack" command works and will convert the "single-photo-stacks" to normal, single images.