Very simple to reproduce.
In grid view (main screen):
– Select a couple of photo (2 or more)
– Let's move theses to quick collection by pressing letter « B »
– all of your selection is in quick collection. press « B » again to remove them.
Switch the grid to the second monitor (by pressing « SHIFT+G »):
– do the same operation, but now press « B » apply only on one photo.
This is do the same with quick collection, stars, color, mark...
In grid view (main screen):
– Select a couple of photo (2 or more)
– Let's move theses to quick collection by pressing letter « B »
– all of your selection is in quick collection. press « B » again to remove them.
Switch the grid to the second monitor (by pressing « SHIFT+G »):
– do the same operation, but now press « B » apply only on one photo.
This is do the same with quick collection, stars, color, mark...