I noticed this when I wanted to find files that are different in two folders and copy those differing files to a third folder.
I'm not 100% sure of the correct intended workflow, but I set folders A and B to be compared and folder C as the destination for the differing files; then I deselected Show Identical Files to only show the differing files and to apply the copy operation to only those.
However, after deselecting Show Identical Files, the operations were otherwise correct and limited to the differing files, but the operation for the first item in the previous full file list was always done regardless.



How to reproduce (way 1):

Compare 2 folders A and B (like in image 1_ok)
Folder -> Choose A for All Items
Start Folder Merge, Simulate It
-> operations seem normal (although the delete-copy operations seem unnecessary when the files are identical in both folders)

then deselect Show Identical Files
Start Folder Merge, Simulate It
-> first item from the full file list always erroneously appears in the beginning of the operations list (see image 2_error)

If B (the destination) is chosen for all items, then the extra first item doesn't appear.



How to reproduce (way 2):

Compare folders A and B and set destination to folder C (like in image 3_ok)
Folder -> Choose A for All Items
Start Folder Merge, Simulate It
-> operations seem normal

then deselect Show Identical Files
Start Folder Merge, Simulate It
-> first item from the full file list always erroneously appears in the beginning of the operations list (see image 4_error)

Similarly, if you Choose B for All Operations and deselect Show Identical Files, the operations list will have the extra first item (see image 5_error).
Same for Auto-Choose Operation for All Items.